My cat likes to suck on her tail....?

I have a kitten who's about 4 months old. Almost every night, she'll find the tip of her tail, and suck on it, sometimes for as long as an hour!!! It's almost like a baby sucking on a pacifier, but I'm not sure if that's why she's doing it. I've examined the tail to make sure there's not any parasites or anything, and it seems harmless, but is this normal??