...When you don't know where to store your shampoo because your bathtub is a de facto lab for your DS2. We have a test tube rack and test tubes (baby soda bottles, courtesy of Portia's suggestion), plastic beakers, funnels, and a rinse bottle crowding every inch of the tub.
DS wanted to have two baths today to keep working on his buoyancy experiments. He also built a nifty sink model out of a funnel, soap dish holder, pipettes, and a beaker, then proceeded to pour water through it to prove to me that it worked.