Soap with nail polish
(peach st Victoria australia)
well you start with getting 2 small blocks of soap and make sure they are exactly the same size colour and all and see if they will notice when you cover one with clear nail polish and put it in there before your victim has a shower and see that they say that there was no bubbles when they wash them self with the soap that you covered the only reason that i said two blocks is because it might take a while to cover with soap it works but before you put it in the shower wait for it to dry.
Damien: You can swap around 2 soaps too. I'd probably cover HALF the soap with nail polish and let it dry. Then see what happens when your victim tries to use it.