Clear Barrier

I sprinted across the slick terrazzo lobby floor, gunning straight for the open night outside! Then—bam! Right into an invisible wall. I bounced flat onto my tail with a comical squeak, staring up in disbelief. Turned out it was just a giant sheet of ultra-clean glass blocking the doorway. I rubbed my nose, hauled the heavy brass key ring upright against the bottom aluminum frame, and gave the pane a firm tap with my knuckles. Solid, crystal clear, and completely locked. Looks like my outdoor track has a checkpoint!