Gears with teeth lock together and push each other around.
Touching gears always spin in opposite directions.
A small gear has fewer teeth, so it must spin many more times to keep up with a big gear. That is a gear ratio!
Turn the red hand crank, or drag any gear, to spin the whole train.
Use ➖ and ➕ to change the small gear's teeth — watch the RPM numbers and the ratio change!