Warning - this puzzle is gross and may be inappropriate for younger audience. If you are under the age of 18, please consult with your parents before reading further.
3 men want to have sex with 1 woman, but they have 2 condoms only. Each of the 4 people has some unique STD which they don't want to transfer to the rest. What can they do?
They can start by putting the two condoms on top of each other and let the first man have sex with the woman. After that the second man can take out the inner condom and have sex with the woman using just the outer condom. Finally the third man can take the removed inner condom, turn it inside out and place it back in the outer condom. Then he can have sex with the woman using the two condoms simultaneously.