When gelatin is mixed with water, it forms a soft gel. The amount of water that gelatin can absorb is determined by the type of gelatin. For example, Type A gelatin can absorb about ten times its weight in water, while Type B gelatin can only absorb three times its weight in water.
If you were to add one cup (237 ml) of cold water to two tablespoons (30 ml) of Knox unflavored gelatin powder, it would expand to about two cups (470 ml) and would be firm enough to hold its shape when cut with a knife. If you then put this mixture into the fridge for an hour or so, the gelatin would set completely and would be like a solid block.