Anode rod
A 'sacrificial' metal rod that rusts so your water heater tank doesn't.
Inside a tank water heater is a metal rod designed to corrode in place of the tank itself — it 'sacrifices' itself to rust. Once it's used up, the tank starts rusting from the inside and its days are numbered. Checking or replacing the anode rod every few years can add years to a water heater's life for a few dollars.