Water pipes have become rather popular as a nice middle ground between pipes and bongs because of their use of water but much smaller size. There is no rhyme or reason behind what material a water pipe should be made out of anymore than there are for bongs or pipes – just that glass is more fragile than others, silicone lightweight and indestructible, etc. If someone wants to have a smoother hit than a pipe in a smaller package than a bong, a water pipe is the way to go!