Matthew can turn doorknobs. He's been able to do so since he turned 3 last year. Closed doors are not barriers to him in our house anymore.
There were a couple of times when he went into Elizabeth's room and woke her up from her nap, much to my annoyance. But sometimes he goes in there only when he hears her over the baby monitor.
Yesterday, he heard her babbling on the baby monitor. She had just woken up from a nap. "Oh, she's up," I said. He immediately ran to her room to go get her. I followed him with my Mommy spycam and captured the giggling siblings on our Flip video camera.