For some reason our bathroom door has 1 1/2 inch (30cm) gap at the bottom. If I shut the door it won't be too long until I see little paws coming under the door. A game ensues where I stick a finger, or toe, under the door and they try to trap it with paws.

One time I was sitting in the bathroom, guess what I was doing, and Fifi started scratching at the door and whining. I opened the door. She started to come in. She blinked twice, shook her head, turned, and ran! They don't so much worry about closed doors after that.