The print head, which is the engine of your printer and which may or may
not be located inside the cartridge, houses hundreds of tiny, delicate nozzle assemblies. Each nozzle assembly consists of a tiny ink chamber, a resistor that controls the flow of ink, walls that guide the ink to the right position, and a nozzle plate with a hole, from which the ink will be sprayed onto the printer paper.

After each time a nozzle fires, a new supply of printer ink is automatically
drawn into its chamber, to be ready for the next time. When the printer is
told by the computer to print a page, the copper circuits on the end of the
ink cartridge send a message to the nozzle's resistor, which then heats
the nozzle's ink supply just enough to cause it to expand and to force a
drop of ink through the nozzle onto the printer paper.

The ink which flows through each nozzle assembly functions as a lubricant and coolant for the nozzle: if there is no ink in the chamber when the resistor turns up the heat, the nozzle assembly will quickly warp and break apart -- the resistor can reach a temperature of hundreds of degrees very quickly! If the print head is allowed to begin this process of burnout (i.e. if the ink cartridge is not refilled or replaced promptly), the damage may range from poor print quality (streaks or lines across the page, bad coloration, light or dark patches on the page) to serious damage to the printer.

It is very important never to attempt to print with an empty ink cartridge. If
in doubt, top off your cartridges frequently (the leftover ink from your inkjet
refill kit can be stored until the next time you top off)