What's up with my cat going crazy over celery?!?

I came back from the grocery store and started unloading on my counter. My cat, Whiskers, jumps up to see what's going on (as usual) and starts going crazy over the celery. He put his scent on it, started rolling next to it, "hugging" it, and getting really frisky. It seemed to have the same effect as cat-nip! It was really cute and funny and I was just curious if anyone has any explanation for this!? : )