What's the coolest trick you ever taught a pet?

I never taught a pet a trick.  They were always entertaining on their own merit. 

I taught my dog how to play Oh Christmas Tree and 3 Blind Mice on a rubber bulb bicycle horn. He dosen't really know the songs  he just responds to hand signals and bites the rubber bulb every time I open and close my hand. The last 2 dogs I had/have were tracking dogs the first dog took a lot of hours to train but the second dog was easy because the first dog taught him every thing he knew before he passed on. Cheers!

The dumbest one I've ever taught was "speak". As if she doesn't bark enough!?!?

Don't know how cool this was but when I used to have cats I would teach them to play fetch with a bottle cap.  I would throw it and they would retrieve it and bring it back to me.  They loved to play and often would bring the bottle cap to me for me to throw to start off.