water rocket

from the GNU version of the Collaborative International Dictionary of English.

• (Bot.) A cruciferous plant (Nasturtium sylvestre) with small yellow flowers.
• A kind of firework to be discharged in the water.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A rocket that uses water as a propellant, which is forced out of the rocket by compressed gas (typically compressed air)