Thermal expansion is the process where materials expand when they are heated. In the context of glass coffee cups, when you pour hot coffee into the cup, the heat causes the glass to expand. However, if the glass isn't designed to handle this expansion (like non-tempered or non-borosilicate glass), it can crack or shatter. High-quality glass coffee cups, however, are made to accommodate this thermal expansion and are safe for pouring hot liquids.