What is it with Kids?!?!? Part 2

Bug is at it again!
We have our large sofa in front of our living room (front room) window. It is pulled away from the window so it doesn't interfere with the curtains or cover the heater/AC vent. Bug loves to play between the sofa and window. He likes to look out the window and play with his cars on the window ledge. I often find his toys back there...no problem. Yesterday, while he was really enjoying being a BIG TURKEY, stuffing TP down the toilet, he also decided it would be lots of fun to open the heater/AC vent and throw stuff down there. Fortunately, Buttercup was there to save at least one thing he was trying to put down there (a "SMILES" pop stick...you know, "if you chance to meet a frown...smile that frown away" with the smiley face on one side and the frowny face on the other). Paper burns real nice when it gets hot!
What is it with kids and vents...or any kind of hole for that matter?