Has your housemate been pissing you off beyond reasonable limits?
Do you want to repeatedly bash their heads against a brick wall until their skulls crack wide open?

Then get back at them by adding salt to their shampoo, shower gel etc. (basically, anything with ionic surfactants) and mix it up very thoroughly - it will clump up and gain about the same consistency as hard-set jelly. Bonus points if it's in an opaque bottle, then it'll be a while before your act of sabotage is noticed.
( The Mock TurtIe ™ --- Thinks you are a cunt, on, Sun 27 Sep 2009, 23:28, 3 replies, latest was 15 years ago)

What happens
if you don't mix it up, but leave them to do it themselves when they massage the shampoo in...?
(Paolo Nutini's Bikini Better than Frederick Fleet's optometrist, Tue 29 Sep 2009, 0:02, Reply)

^ this
is genius.
( sandettie light vessel automatic New Twitter - @bollocksreally, Wed 30 Sep 2009, 11:17, Reply)

Try it....
...and report back.
(Wallilay has been bored of this shit since, Tue 13 Oct 2009, 12:54, Reply)