Downstems are one of several types of interchangeable glass attachments on the market, something you may need to buy down the line. A downstem is the glass piece that connects the bowl that you pack your dry leaf in to the water source in the water pipe.
Downstems make the whole process function, so most bongs and bubblers will come with a downstem, however accidents happen. Glass breaks, durable and sometimes things get lost. So, having the option to replace your detachable downstem is both convenient and necessary!
Many downstems, as well as the rigs and bongs they are attached to, are made of borosilicate glass. You might be familiar with borosilicate under the brand name Pyrex. This type of glass is resistant to thermal shock, which has obvious benefits for smoking and extremely difficult to break, which is never a bad investment in our opinion.