Sink or float?

I love when learning happens during every day life.  My son was playing in the bathroom sink with three toys.  All of them were Percy from Thomas the Tank Engine.  He had one that was a plastic bath toy, one that was a solid wood train piece, and one that was a plastic matchbox car size.  He decided to fill up the sink to see which one(s) would sink or float.  The plastic bath toy floated and the other two sank.  So then we had a great discussion on WHY each floated ir sank.  I asked lots of probbing questions like "what are they made of?". Then we had a great discussion on solid objects.  He determined that the bath toy floated because it has air in it.  Since this "experiment" he has been playing with his other toys deducing if they would float because they also have air in them. :)