If you have and want to patch/ seal a cracked glass or you want to make anything from scratch where you seal the glasses together this is what you need. Silicon is used a lot for sealing things as well as seal or patch glass because it dries fast. It is ideal for sealing aquarium glasses. It is suitable for sealing gaskets especially some other types of weather-stripping, outside of fireplace doors, shoes and boots of hard materials. It can also be used to bond some types of ceramic, porcelain, wood, aluminum, stainless steel, cultured marble, fiberglass, granite, tile, some hard rubbers and plastics. The gun helps to press the silicon out skillfully. Optionally without the gun you have to really really force the silicon out from the container which might make the user spill more than enough silicon than needed. It is not just better to use silicon without the gun (pointed mouth) for a neat job. Silicon come in different color like white, black but this is a clear/ transparent silicon, ideal for glass uses because one can likely see through this silicon gel and doesn't appear as obvious as other silicon would on glass.