* Salt lowers the freezing point of water: Salt is a solute that dissolves in water. When salt dissolves, it disrupts the water molecules' ability to form a solid structure (ice). This lowering of the freezing point means that the ice will melt at a lower temperature than it normally would.

* Sugar doesn't significantly lower the freezing point: Sugar, while it dissolves in water, doesn't have the same effect on the freezing point as salt does. It doesn't disrupt the water molecules' ability to form ice as much.

In short: Salt makes ice melt faster because it lowers the freezing point of water, causing the ice to melt at a lower temperature.