Why doesn't Kitty Pryde fall through the floor when using her special powers to phase? If she can move through walls, etc, surely she should just keep falling down.
While phasing, she does not physically walk on surfaces, but rather interacts with the molecules of air above them, allowing her to ascend and descend, causing her to seemingly walk on air.
One could extrapolate that she interacts with other molecules to control how much she would phase through the ground, when she does so.
It has been shown, in a few stories, that she can walk high in the air, but she has to concentrate in order not to fall.
In Uncanny X-Men #143 (1981) it shows her "using her phasing ability to literally walk on individual molecules of air":