This is a classic Martin Gardner puzzle.
See the picture - the four matchsticks represent a martini glass and the match head represents an olive. Move just two matches so that the glass is reformed but the olive is outside of the glass. The olive has to stay where it is. The glass may be turned to the left or right or be upside down at the finish.
The upper right diagram is wrong because the olive is still in the glass. The lower right diagram is wrong because three matches were moved.
Move the horizontal match right by half a matchstick, and then move the upper left-side match to the bottom right.