A secret because I've never actually told this to anyone... The rainbow bubble bar from Lush is my favorite of all the products they offer, because it really makes a lot of bubbles, but also because it cleans my tub while I'm having a bubble bath. I noticed this the first time I used it, that afterwards the sides of my bathtub were sparklingly clean. It might take a little scrubbing, especially if you have hard water, but your tub will be clean with surprisingly little effort. Probably has something to do with the fact that there's lemon oil in them. Also makes me think of how, in Japan, people scrub their bathtubs after every bath so the tub doesn't get funky.