Where did he learn to climb??

I'm at work and just talked to Ryan - he is home with Ethan and he went up one step. Ryan stood behind Ethan to see what he would do, and the little booger kept climbing and went all the way up. It's about 15 steps. I can't believe it!! I guess we definitely need to leave a gate up all the time now. Walking and climbing the stairs all in one week... where has our baby gone??