Some of life's arguably greatest mysteries centre around tickling - why we can't tickle ourselves, why tickling evolved, and why only some parts of the body are ticklish.
But there's one less mystery now - rats are ticklish and they have a lot in common with people.
In a study published in the Science journal on Friday, researchers Shimpei Ishiyama and Michael Brecht experimented on tickling rats.
And it found depending on their mood, the furry critters either loved it - or hated it.
Just like humans, if they're already stressed out, the rats were a lot less likely to enjoy the tickle monster.
But for those in a happier mood to start with, it was an enjoyable experience.
The rats would sometimes jump for joy, called freudensprünge, and even laughed, in their own way - it's a higher-pitched call, which can't normally be heard by humans.
The researchers examined the rats' brain activity during tickling and found stimulating the same area, without tickling, also made the rats giggle.
The rats began getting tickled when they were young, to get them used to the sensation. It had the bonus effect of the rats seeking out the tickler unprompted - further proof it's something they actually enjoyed.
"Although it was a very bold idea in the beginning, experiments show that rats are enjoying it," Dr Ishiyama said.
And in case you were wondering - the back is the rat's most ticklish spot.