Big Idea: Frequency means the amount of vibrations per second (also called rate). High frequency means many vibrations per second, low frequency means fewer vibrations per second.
The demonstration with the spinning Lego gears shows how the piece of paper moves up and down (vibrates). The big gear has many teeth so it causes many vibrations per second, the smallest has the fewest vibrations.
Connections:
Can you make a connection to a tuning fork?