Water Rockets!

from wikipedia:

A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel—the engine of the rocket—is usually a used plastic soft drink bottle. The water is forced out by a pressurized gas, typically compressed air.

The term "aquajet" has been used in parts of Europe in place of the more common "water rocket" and in some places they are also referred to as "bottle rockets" (which can be confusing as this term traditionally refers to a firework in other places).

Water rocket engines are most commonly used to drive model rockets, but have also been used in model boats, cars, and rocket-assisted gliders.

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