Rats laugh and we discovered this by tickling them.
Rats are very social creatures and thus often wish to play, though it was noticed that when playing they will make a sound distinct from their normal vocalisations.
It was queried if this sound was comparable to laughter in humans. So to test this a group of scientists performed the one sure fire method to make something laugh, in order to compare, they tickled them.
The rats emitted the exact same sound as when they play, thus proving that they were indeed laughing. What only solidified this result further was when it was noted that if the same rat was tickled every day, the rat will start making that sound when they see you. Anticipating playing with you.
This may be the most adorable thing ever.