Take some stuffing out of the pillow. Poo in the pillow. Sew it back up. Put it back on Kate's bed. The poo will soak in. She washes it. It gets worse. Kate is sad.
Caitlin: Hey guys! You know what would be really funny?
Sheila: No, what??? I'm super bored!
Caitlin: Let's poo in Kate's pillow and put it back on her bed!
Sheila: OMFG that's such a good idea. When she goes to wash it the smell will get worse!!!!
Caitlin: Totes. Poo pillow. You know what else is fun?
Sheila: No, what? OMG!! WHAT?
Caitlin: Doing your taxes drunk.