Bongs are often either glass or plastic that use a bowl, stem, and water to produce smoke. Most glass bongs are made from heat resistant borosilicate glass, allowing the bong to withstand repeated use and heat exposure without breaking. After the bowl has been packed and water has been inserted into the bong, the substance is lit and the smoke is drawn through water to produce a smoother smoke than other methods of smoking do. To smoke a bong, the smoker must inhale in the bong so bubbles containing smoke begin to come from the stem. Once the bong has a fair amount of smoke built up, either the carb is uncovered or the stem is separated from the bong, allowing the remaining smoke to be inhaled.