You’re in trouble

The vet says that Yogi needs to have his urine tested for kidney function.
We have to use special cat litter in a tray and shut him in with it until he goes.
The urine will then float on top of litter to be syringed off and brought back to the vet within 12 hours.

Are you taking the piss? I asked him.
No, but you will be, he replied.