I've been thinking about soap. About: how odd it is that we rub a slimy chunk over our wet hands before rinsing the slime off. About how normal it seems the more odd it seems. What could be more normal than trying to stay clean, and what could be more relevantly useful than soap?
But of course, soap is only normal because we have always known it. On distant planets, other intelligent creatures do all sorts of things we would never dream of. If you'd never used it or heard about it, would you dream of soap?
Perhaps not, but you might dream of cleanliness. Do soap-like substances appear all over the universe? Life has been discovered in the frozen oceans of Europa. Soap would dissolve there. What would life be like in a liquid atmosphere?