Every night, the boys argue over who gets which footstool to stand on to brush their teeth. The two plastic footstools are completely identical, bought at the same time, in the same place, are the same exact color, weight, shape, without any identifying marks (like sticker residue or scratches) to tell them apart.
So you know what they fight over?
Who gets the dry footstool.
See, climbing out of the bathtub, Julian still needs a footstool to step down onto. Dave sits on one footstool, and Julian's wet feet touch the other, thus creating a temporary distinction. And a few drops of water is enough to ignite a major row over the dry footstool every night.
I'd be tempted to quickly dry off the wet footstool before the argument erupted. But why? They'll simply find something else different about the footstools to argue about. They're that dedicated.