Name something funny and/or nasty you've done to your FILs

It could be nice, it could be nasty. It should be funny. I'll start.

We used to have a couple of small black shark looking fish in the fish tank in the front of the office. They were about 6 inches long and really looked like small black sharks.

One Died.

I fished it out of the tank with the net and brought it to the toilet to flush... except, I didn't flush. I left it there for FMIL to discover.

She came in about an hour later and I forgot about my little surprise until I heard her YELP from the bathroom! I was doubled over in laughter.

ETA: This is the fish:

GOOOD ONEEEEEEEEEEEEEE!!!!!!!!!!!!

I should start doing stuff like that to FSIL's and FMIL
mostly I am just annoyed and angry

tormenting them a little bit will def entertain the hell out of me

any suggestions? I like being sarcastic and passive aggressive

ETA: Becca that is really hilarious. I wish u saw her face when she just saw it in there belly up haha