As soon as Spencer began crawling he discovered that the living room was a lot bigger than we had initially let on. He discovered that the dog has bowls with food and water and if he hits them just right they will tip over. He has also realized that Prince gets to come in and out as he pleases because of a doggy door!
The first time he crawled to the door he sort of just looked at it. The next few times he touched it and now...well now he pushes it in and out. He hasn't crawled thru it but I imagine it is only a matter of time until that happens.