I think I've talked about the tutorials in Half-Life 2 before, but the rollermines are a prime example of how Valve teaches you how to play the game while you're playing the game.|
In the Black Mesa chapter, shortly after you get the gravity gun, there's a highly enjoyable sequence where you get to play catch with Dog, using a deactivated rollermine (though you don't know it's a rollermine at the time, it's just his ball).
Here, a few chapters later, you encounter the real, dangerous rollermines that like to attach themselves to your car and give you electric shocks. And then you realize that the catch with Dog wasn't just teaching you how to use the gravity gun, but also that you can use it against this particular enemy.
Of course, I was even more clueless than Frohman in this instance. The first time I played, I was driving slowly along the coastal road and the first rollermine comes rolling along at me. I think "Hey! It's Dog's ball! Dog must be around somewhere!" I get out of the buggy, grab the rollermine with the gravgun, and shoot it a little bit up the road, fully expecting Dog to come clomping out and pick it up. He didn't. The ball just slowed, stopped, and came rolling back. I didn't pick it up this time and it rolled up to me and it JUMPED AT ME AND BIT ME AND HURT ME. Not my most shining moment.