Realizing that Belle is literally the perfect companion.
“Why do you run so much?” Once they were inside the TARDIS, the door safely closed behind them, Belle collapsed to the ground. Her side ached and she felt like she might need to drink an entire swimming pool of water before she was hydrated again.
“Generally because things are chasing us, dearie.” He busied himself with the console, but she knew him. The movements he made were not the confident ones that spoke of a destination chosen, but the small strokes and minor fidgets that meant he needed to keep himself distracted. That, or as Belle had suspected before he was petting the thing.