Sean trips, and his knife falls out of his pocket. He notices that the knife floats in mid-air and checks his compass to find that it isn't working, meaning there is a magnetic field in the area. He travels to the other side of a large opening using large rocks suspended in air, with many close calls during his journey.
This clip takes apart a coin sorter and explains how the inside of it works. It uses electromagnets, currents, and disturbances in magnetic fields to determine the amount of metal in the coin.
The Doctor hides himself in an elementary school to investigate an alien attack. As the teacher, he asks his students various physics questions to understand how much they know. One student, Milo, proves he's unusually smart and triggers the Doctor's interest.