What makes a wheel turn?

I won't go into all of that, but I do know what can make it stop... and so does my 2 yr. old son:

I was grocery shopping at a local store with my 9 month old in the seat facing me, my 4 yr. old daughter walking beside the cart and my 2 yr. old under the basket (with plenty of room to sit up). Suddenly the cart stops and my son starts to cry. My first glance was at his shoes. He is notorious for scooting forward and dragging his feet in front of the buggy. Or pulling a shoe off just for the fun of it.

Both shoes were on and up off the floor. So what stopped the buggy? A little rock or pebble? Maybe some food particle someone had dropped? But why would Matthew be crying? I knelt down beside him to investigate and he raised a pitiful looking index finger. It was black! What had he done????! Curiosity had gotten the best of him as he reached down to "stop" the wheel from turning. And it had worked! He's not really any worse for the wear, although he does have a small bruise just under his fingernail, but I don't think he will be trying that trick again any time soon!