A great day teaching.

Taught my students about the velocity selector in AP Physics, where the electric force is balance out by the magnetic force. It is a thing of beauty. Get two parallel metal plate and put a big battery across it, you have instant E-field. Put a magnetic field perpendicular to it. If charged particle goes to fast, magnetic field curves it's path. If it is too slow, electric field curves it the opposite direction. If the speed is just right, v=E/B, it goes through undeflected.
We then talked about the path of the particle once it left this region and went into a magnetic field. It has a constant velocity. As a result, R=mv/qB (R=radius, m=mass, q=charge, B = magnetic field), the only variable being the mass. Different masses mean different radii. You could use this device to find out what isotope by percent were in a sample. Or it could be used to separate U235 from U238.
The Goodyear plant in Chilocothe, OH used this method to enrich uranium. It seems that everyone want's to use centrafuges today.
In robotics I got my students to program their robots to make a perfect square, until they ran into an obstacle. The used infrared leds to modulate a signal. If the modulated reflected signal were picked up, an obstacle was detected. The program would go into an avoidance subroutine, after which it would go back to making a square.