A gas cylinder is a pressure vessel used to store gases at above atmospheric pressure. When the gas in the cylinder is ready to be used, the cap is taken off and a pressure-regulating assembly is attached to the stop valve. This attachment typically has a pressure regulator with upstream (inlet) and downstream (outlet) pressure gauges and a further downstream needle valve and outlet connection. If the gas is heated, it will expand and push the piston up, thereby doing work on the piston. If the piston is pushed down, on the other hand, the piston does work on the gas and the gas does negative work on the piston. This is an example of how work is done by a thermodynamic system. For gases that remain gaseous under ambient storage conditions, the upstream pressure gauge can be used to estimate how much gas is left in the cylinder according to pressure. For gases that are liquid under storage, e.g., propane, the outlet pressure is dependent on the vapor pressure of the gas, and does not fall until the cylinder is nearly exhausted. The regulator could be adjusted to control the flow of gas out of the cylinder according to pressure shown by the downstream gauge. The outlet connection is attached to whatever needs the gas supply, such as a balloon for example.