My son built a small project where he tried to control the speed of a computer case fan with a (variable) resistor, shunting a % of the power to ground. Didn't

My son built a small project where he tried to control the speed of a computer case fan with a (variable) resistor, shunting a % of the power to ground. Didn't work very well. The fan either ran full speed or not at all. So he used a 555 chip to turn the setup into pulse-width modulation, with the variable resistor (pot) adjusting the pulse width. Worked like a charm.