Inmates have nothing but time. As trainers, we constantly push that inmates have 24 hours a day to plan escapes.
They also have plenty of time to plan pranks.
As pranks go, replacing an inmates lotion with a depilatory (IE: Nair) is pretty funny. They step out of the shower, lotion up, and let the burning sensation begin!
The perpetrators got out one line of "Who wears short shorts" before the beatdown began.