The Science of a Heat Vent

I've been attempting, unsuccessfully, to teach my daughters the purpose of a heat vent and how it works.  Here's a picture of a heat vent in our house.

In the typical home you have a furnace that warms air and then a large fan pushes the warm air through ducts and up through the vents that are scattered around the house.  In order for a room to warm, it needs an unblocked heating vent.  If the vent is blocked, the air just re-directs to a different vent in the house.  I've tried telling this to my daughters over and over, yet they still insist on stacking toys and/or boxes on the heating vent in their toy room when they clean.  As a result, the room is usually cooler than other rooms in the house.  They then complain about it being cold in there.  Well, duh!!!  

I guess this is one of these cases where they just need to learn the science the hard way...as in being cold!