Sonic pen

The Sonic pen that Miss Foster had worked much like the Doctor's sonic screwdriver, but it looked like an ordinary pen. The Doctor activated it whilst pointing it directly at his own Sonic Scredriver, creating a sonic feedback loop. Before doing so, he asked Miss Foster whether she was aware of what would happen when two sonic devices of the same type were used in such a way, implying that her pen and his screwdriver were manufactured at the same place.