Acquiring a Tire Stem

Tire stores usually have lots of used stems lying around, since the old stems are discarded when new tires are installed. These shops are very frequently staffed by young fathers who will think building a water rocket sounds awesome and will be eager to give you as many stems as you need. Make sure to tell them that “broken stems are okay”—a broken stem is one that’s had its bulbous end snapped off (you can see this round end on the top of the stem on the left of Figure 20-3), and it will work just fine for this project. Alternatively, head to a bike shop, where lots of leaky bike tire inner tubes are probably on hand. A car tire stem is a separate unit from the tire and the perfect dimensions for a soda bottle water rocket, while bike tire stems are integrated with the tube and will need to be cut loose.