Welcome to MindCipher, a social repository of the world's greatest brain teasers, logic puzzles and mental challenges.

4 buttons and a light bulb

There are two rooms, in one room there is a light bulb, and in the other there is a round table, with 4 buttons arragned in a square on it. Each button may be on or off, and the light bulb will be on only if all the buttons are on. Every time you leave the buttons room to the light bulb room, the table with the buttons rotates randomly. How, without knowing the buttons' initial state, you can make sure the light bulb will turn on?


  • The buttons all look the same, and you can't distinguish between them.
  • You can't distinguish between a turned on button and a turned off button, the only way to gain information on the buttons' state is the light bulb.
  • You can't see the light bulb from the buttons room, you need to enter the light bulb room to see it.
  • The light bulb doesn't produce any heat, so you can't feel if it was on before by touching it.
see solution


We can't keep track of the buttons' state (it changes every time since the table rotates). So, we will divide all the states to six groups, with the condition that in each group, if you rotate a state, you will get another state of the same group. (If you are familiar with equivilance relations, we are dividing the states to equivilance classes, with the relation "x can be rotated in order to get y"). These are the groups we get:

  • A - all the buttons are off
  • B - exactly one button is on
  • C - two buttons are on, and they are near each other
  • D - two buttons are on, and they are opposite to each other
  • E - three buttons are on
  • F - four buttons are on

We know that if previously you had a state in group X, when you go to the light bulb and return, it will still be an X state, since the states in X can rotate to each other. So, we can keep track of the groups of states, instead of the states themselves.


We can eliminate those groups, one at a time, while keeping track of the current possible state-groups at each step:

  • We start with either A, B, C, D, E or F (all possible states).
  • 1 - Eliminate F: Check if the light is on, if it is your'e done, if not, the state can't be in F, we remain with A, B, C, D, E.
  • 2 - Eliminate A:
  • 2.1 - Press all the buttons, A will turn into F, B will turn into E, C to C, D to D, and E to B. now we have B, C, D, E, F.
  • 2.2 - Eliminate F: Check if the light is on. if not, we remain with B, C, D, E.
  • 3 - Eliminate D:
  • 3.1 - Press two buttons that are opposite to each other, B or E will remain B or E (they have odd number of buttons, and by pressing two buttons you remain with and odd number), C will remain C, and D will become either A or F. we now have A, B, C, E, F.
  • 3.2 - Eliminate A and F (like steps 1, 2): check if the light is on, if not, press all the buttons and check again. we now have B, C, E.
  • 4 - Eliminate C:
  • 4.1- Press two buttons that are near each other, B or E will remain B or E like before, C will become either A, D, or F. we now have A, B, D, E, F.
  • 4.2 - Eliminate A and F: check if the light is on, if not, press all the buttons and check again. we now have B, D, E.
  • 4.3 - Eliminate D like step 3:
  • 4.3.1 - Press two opposite buttons, B, E will remain B, E, and D will become A or F. we have A, B, E ,F.
  • 4.3.2 - Eliminate A and F: check if the light is on, if not, press all the buttons and check again. we now have B, E.
  • 5 - Press one button, B will become A, C, or D, and E will become C, D, or F. we have A, C, D, F.
  • 6 - Eliminate A, F: check if the light is on, if not, press all the buttons and check again. we now have C, D.
  • 7 - Eliminate D:
  • 7.1 - Press two buttons that are opposite to each other, C will remain C, and D will become either A or F. we now have A, C, F.
  • 7.2 - Eliminate A and F: check if the light is on, if not, press all the buttons and check again. we now have a C state.
  • 8 - Eliminate C:
  • 8.1- Press two buttons that are near each other, C will become either A, D, or F. we now have A, D, F.
  • 8.2 - Eliminate A and F: check if the light is on, if not, press all the buttons and check again. we now have a D state.
  • 8.3 - Eliminate D:
  • 8.3.1 - Press two opposite buttons, D will become A or F, so we have A, F.
  • 8.3.2 - Eliminate F: check if the light is on, if not, we must have an A. now we can press all the buttons to get an F, and wer'e done!



it was really good watch this video https://youtu.be/tWztTPvtVbo


seahawks jerseys, long champ, philipp plein clothing, bulls jerseys, okc jerseys, los angeles lakers, swarovski jewelry, coach outlet usa, prada tracksuits, toronto raptors jersey, air max, kate spade handbags, adidas.se, woolrich womens jackets, tommy hilfiger online, coach outlet online, chrome hearts rings, coach factory outlet online, saints jerseys, tommy hilfiger outlet, polo outlet, air max thea, oakley sunglasses outlet, nike shoes, toms outlet online, gucci store, cheap oakley, nike.dk, ralph lauren factory store, rolex, toms shoes outlet, hollisterco, stone island, spurs jerseys, mbt womens shoes, mcm bags, converse shoes, burberry outlet store, polo ralph lauren outlet, hilfiger outlet, ralph lauren outlet online, prada, moncler women jackets, broncos jerseys, michael kors, prada handbags, nike factory, adidas superstar, true religion jeans women, browns jerseys, michael kors uhren, hermes outlet, asics outlet, roshes, colts jerseys, the north face, oakley sunglasses, zapatillas nike, belstaff jackets outlet, designer handbags, replica watches, guess bags, tory burch, hogan, coach factory outlet online, fred perry outlet, replica watches, timberland boots, swarovski crystal, ray ban sunglasses, air max 2014, nike air max, nfl jerseys, bills jerseys, converse chucks, coach usa, kate spade outlet, northface, ralph lauren, portland trail blazers jersey, vikings jerseys, kate spade, polo ralph lauren, washington wizards jersey, omega watches, new orleans pelicans jersey, tods womens shoes, nike.se, marc by marc jacobs, polo outlet store, michael kors, atlanta hawks jerseys, mavericks jerseys, red bottom, coach usa, nike roshe run, belstaff mens jackets, marc by marc jacobs, cheap michael kors, rolex watches, hogan mens shoes, bengals jerseys, polo ralph lauren outlet online, toms shoes outlet, michael kors australia, puma shoes outlet, north face outlet, true religion outlet, nike roshe run, vans outlet, burberry outlet, north face jackets, oakley, polo ralph lauren, birkenstock deutschland, ray bans, nike free run, supra shoes, nike air force, tory burch handbags, givenchy shoes outlet, new balance, polo outlet online, michael kors outlet online, retro jordans, sacramento kings, ralph lauren, chi flat iron, polo ralph, cheap jerseys, ferragamo, 49ers jerseys, patriots jerseys, handbags outlet, nike mercurial vapor, chiefs jerseys, north face backpacks, kate spade bags, hilfiger online shop, eagles jerseys, ray bans, nike roshe run, swarovski australia, titans jerseys, new balance shoes, michael kors outlet online, mcm handbags, ralph lauren outlet online, versace outlet, nfl jerseys, fitflop shoes, new balance outlet, cheap oakley sunglasses, prada outlet, dsquared2, lac jersey, cowboys jerseys, jordan shoes, montblanc, pandora canada, cheap jerseys, michael jordan, pandora jewellery australia, iphone 4s cases, michael kors, true religion, polo ralph lauren, adidas shoes, nike air max, cheap ray ban, red bottom shoes, longchamp handbags, under armour shoes, oakley, barbour jacket outlet, orlando magic jersey, belstaff womens jackets, easton bats, bears jerseys, longchamp, north face, roshe run, ferragamo, redskins jerseys, chi hair, sunglasses for women, adidas online shop, nj jerseys, fendi, giants jerseys, rolex watches, hollister clothing, nike huarache, azcardinals jerseys, salomon schuhe, adidas superstar, burberry outlet store, nike free, louboutin shoes, ralph lauren, thomas sabo, bcbg max, nike huarache, gucci shoes store, michael kors outlet, steelers jerseys, nba jerseys, jordan retro, nike soccer shoes, coach factory online, dsquared2 outlet, pandora charms, soccer shoes, air jordan shoes, michael kors handbags, replica watches, nike store uk, burberry, nike air max, new balance, abercrombie, purses and handbags, juicy couture, true religion outlet, chargers jerseys, new balance, oakley sunglasses cheap, asics gel, under armour outlet, polo ralph lauren, ralph lauren outlet, burberry outlet online, salvatore ferragamo, ray ban uk, true religion jeans outlet, reebok outlet, michael kors handbags, lions jerseys, tommy hilfiger, michael kors bags, nike fu?ballschuhe, cavaliers jersey, babyliss flat iron, minnesota timberwolves jersey, air max 90, ed hardy clothing, jimmy choo outlet, the north face outlet, versace, converse outlet, thomas sabo jewellery, rayban, swarovski crystal, hogan sito ufficiale, coach bags outlet, 76ers jerseys, coach factory outlet, timberland outlet, falcons jerseys, timberland outlet, uggs outlet, burberry outlet online, rayban, celtics jersey, michael kors handbags, celine outlet, nike air max thea, longchamp taschen, ray ban outlet, buccaneers jerseys, g by guess, ferragamo shoes, fred perry polos, ray ban outlet, timberland, bottega veneta, cheap michael kors, ravens jerseys, oakley sunglasses, red bottom, ferragamo shoes, airmax, levis jeans, relojes, utah jazz jersey, tommy hilfiger canada, ralph lauren polo, veneta, ray ban pas cher, pandora charms, uhren shop, valentino outlet, christian louboutin outlet, pandora bracelet, toms outlet, prada shoes, the north face, oakley sunglasses, rayban, mcm handbags, cheap basketball shoes, skechers canada, air jordan retro, north face canada, panthers jerseys, nba jerseys, michael kors v?skor, bcbg max azria, nike running, jack wolfskin outlet online, denver nuggets jerseys, gafas oakley, lacoste shirts, eyeglasses stores, indiana pacers jersey, jimmy choo, nike roshe run, fivefingers, ray ban sunglasses, hollister online, nike free 5.0, hollister co, philipp plein, guciheaven outlet, giuseppe zanotti, c&c beanies, hornets jersey, ray ban occhiali, reebok, michael kors, oakley frogskins, juicy couture outlet, nike blazers, cheap nfl jerseys, hollister co, burberry online shop, birkenstock, christian louboutin shoes, michael kors outlet online, nike air huarache, cheap jack wolfskin, detroit pistons, barbour jackets, purses and handbags, moncler mens jackets, burberry sale, michael kors uk, nba jerseys, barbour jackets, cheap oakley sunglasses, lunette ray ban, fidget spinner, cheap jerseys, hermes birkin bag, nike air max, toms shoes outlet, michael kors outlet online sale, prada outlet, coach outlet online, nike tn, toms.com, ny jersey, thomas sabo uk, raiders jerseys, air max, ecco shoes outlet, hoodies store, warriors jerseys, burberry, calvin klein outlet, michael kors, burberry, michael kors purses, cheap shoes, ugg australia, easton bats, nike.com, miami heat, mcm backpack, polo ralph lauren, ray ban wayfarer, nike running shoes, christian louboutin, michael kors bags, adidas sneakers, oakley sunglasses, coach bags outlet, michael kors outlet online sale, ralph lauren polo, mizuno, pandora charms, replica rolex, jets jerseys, lacoste, dolphins jerseys, swarovski online shop, armani outlet online, adidas yeezy shoes, texans jerseys, coach shop, north face jackets, ralph lauren online shop, supra footwear, barbour mens jackets, mizuno running shoes, puma online shop, converse sneakers, nike air max, burberry handbags outlet, burberry outlet store, jordans for sale, nike store, coach black friday, vans schuhe, vans shoes, hollister, coach outlet sale, giuseppe zanotti sneakers, oakley sunglasses cheap, dansko shoes outlet, mont blanc pens, longchamp outlet, nike shoes, hugo boss outlet online, ipad mini cases, nike air max, michael kors handbags, www.tommyhilfiger.nl, cheap oakley sunglasses, jordan shoes, barbour womens jackets, jaguars jerseys, burberry sale, adidas, soccer shoes outlet, coach outlet store, rams jerseys, milwaukee bucks jersey, michael kors outlet, birkenstock taschen deutschland, adidas zx flux, montre homme, cheap jerseys, skechers womens shoes, ray ban wayfarer, louboutin, air max 2014, nfl jerseys, nike schoenen, oakley sunglasses outlet, cheap nike shoes, mcm handbags, packers jerseys, oakley sunglasses, suns jersey, hermes belt, ray-ban sunglasses, skechers outlet , rolex replica, memphis grizzlies jersey, cheap jerseys, houston rockets, swarovski, Herm├Ęs Pas Cher, longchamp handbags, pandora, oakley sunglasses, true religion jeans men, ray ban zonnebril, asics, nike running shoes, longchamp, coach bags outlet

Check out other puzzles:

Like this? You might also like:
No if() Only abs()
Bug's Eye
Power of 2 (Microsoft Interview)
Submitted by
almost 2 years ago
Difficulty 8.0 ?

logic reasoning equivilance classes