Always make sure you can hear your child at all times.
If I had remembered this very important rule, I probably wouldn't have turned up my music just a little louder while I was cleaning the kitchen. I may have heard him removing the bedding from his mattress, then removing the mattress itself, and turning his bed frame into a very nice train table. Nevermind, that there's an actual train table lying underneath where the mattress is now. It would've been much too easy to just use that one.
