Something about my children leaves me quite speechless. We keep a plastic cup in the bathroom for mouth rinsing after brushing our teeth. Sometimes it falls off the sink. But the kids don't bother picking it back up after they've dropped it, they go to the kitchen to get another one! So if i don't clean the bathroom for a while, we end up with a collection of cups on the floor.
It can't be out of laziness that they don't pick it up. Because it would take less energy to pick it up and give it a quick rinse than it would to go to the kitchen to get another cup.

Go figure!