The Bicycle Water Pump Modification is a toy/tool designed not only to expose children to the mechanics of a closed system pump (like the ones found in heart/lung machines), but also, to give them the opportunity to experiment with pressurized fluid systems. The pump is quite simple; a length of garden hole is threaded onto the pump, and the other end is immersed in a bucket of water. The user then attaches the pump in place of the back wheel on their bike. Pedaling the bike forces water along the flexible tube, and generates pressure. This toy is ideal for defending a clubhouse, where a large volume water cannon would be advantageous. It could also be used for experiments that required the raising of water to a higher level-like a gravity water pipe course for instance. The best asset of this toy is its versatility. This toy could be used by anyone who knows how to pedal a bike, although it would require parental help to switch out the rear wheel.