Dear Malcolm,

I was surprised the first time I saw a human transform into a tickle monster. They curled up their fingers and rhythmically poked someone else’s armpits. That is something a cat would never do.

I asked my friend John Wright about it. He’s a retired psychology professor at Washington State University.

I learned that there are two kinds of tickling.

The first is the light tickle you feel when someone gently brushes your skin. Like when someone uses a feather to barely touch the inside of your arm. Scientists call that knismesis.

It turns out you can totally tickle yourself that way. But the feeling will be much more tickly if someone else does it.