I was really really struck by this the other day. My middle child, who is almost four, had a few bits of Lego and my top fave bit of Lego kit, the taker-aparterer. But one 8x2 standard-height brick had an 8x2 flat bit on top of it, and she couldn't separate them. She asked me to help, and I did. Then came the bit that struck me.
I said, "Here you go - shall I show you what I did?"
And she said (this was the first part I noticed), "Yeah."
And I said, "Look - you were pulling them apart the long way, and the top one bent in the middle but didn't come away. I put this thing on the other side, and pulled them apart the short way."
And she said "Ohh! I see!"
It was really quite something, at least for me.