The Invisible Wall

The patio door was firmly shut, but that wasn't going to stop my patrol! Outside, fat drops of rain were darting down the glass pane like scurrying silver bugs. I flattened my belly against the floorboards, whispering a low murmur as I crept closer. One, two... pounce! *Thump!* Both my front paws hit cold, smooth glass. The water droplet just kept sliding right past my nose on the other side! I gave the pane a stubborn headbutt, leaving a round little smudge, then shoved my paw into the bottom metal track to see if I could slide the whole wall open. Locked tight, but I'll figure this barrier out!