Most consumer inkjet printers, such as those made by Canon, HP, and Lexmark (but not Epson) use a thermal inkjet; inside each partition of the ink reservoir is a heating element with a tiny metal plate or resistor.
The ink also acts as a coolant to protect the metal-plate heating elements − when the ink supply is depleted, and printing is attempted, the heating elements in thermal cartridges often burn out, permanently damaging the print head.