At 21 months Sam has almost completely conquered our house, physically. He can climb up and down the stairs himself. I rarely keep the gate closed anymore, because he's more careful on the stairs when I'm not with him. I love this milestone. He can open all the doors in the house. I don't love this milestone as much. It necessitates keeping the deadbolts locked at all times, which is easy to forget. So far I haven't had to chase him down the street or anything, but I'm guessing with such a helpful big brother he'll figure out how to unlock the deadbolt before too long. *sigh*
He can climb onto all the chairs, and because of the aforementioned helpful big brother he figured out that if you push a chair up to a counter you can reach what's up there. Ben didn't start doing that until he was at least 3, so this is new territory. We use folding chairs in the kitchen, so when we're not using them they're now folded and leaning against the wall. Luckily, Sam hasn't put together that he could push a chair from the dining room into the kitchen to achieve the same result. Let's hope it stays that way for a while.
And just yesterday I witnessed him open the refrigerator. That kind of terrifies me. There's so much horribleness that could come from that skill. I can't remembered when Ben mastered that, but it was never a problem with him. He only used the ability for good, not evil. Hopefully Sam will learn that from him, too!