If you could make your pet understand one thing, what would it be?

I want my cat to stop shitting in my bathtub. That little bastard does it like one a month. He probably hates sharing the litter box with the other cat, or maybe they were using it and hissing at him. I get that, maybe it was an emergency that comes up every month. I know when there’s company over I don’t even like using my upstairs bathroom, so I get that. It’s my own home, but I don’t want to blow it up with guests around. Its up to guests to blow up my bathroom and embarrass themselves for me!