Al's soap bubble wall of Protego popped silently and vanished, as he lowered his wand.
"Hm, all right," he said, considering.
He glanced at Dad, glanced at the door. It didn't make much logical sense to protect the doorway from Dad, but he decided to try it anyway. He measured the exact ten steps (what he found to be the optimal distance for casting a shield charm) away from Dad, turned around, turned his back to the doorway, raised his wand, murmured the spell, and resurrected the Protego soap bubble, staring through it at Dad with all the determination of a newly appointed doorkeeper.