To launch a simple rocket, the untied opening of an inflated balloon is released. The elasticity of the balloon contracts the air out through the opening with sufficient force and the resulting pressure creates a thrust which propels the balloon forward as it deflates. It is usual for the balloon to be propelled somewhat uncontrollably (or fly in and unstable centre of mass), as well as turbulence that occur in the opening as the air escapes, causing it to flap rapidly and disperses air outwards in random direction.