The reaction between the lemonade powder and water occurs on the surface of the powder particles. By moving through the water, the lemonade powder particles comes into contact with water molecules. Stirring induces the lemonade powder to move faster through the water. It actually increases the kinetic energy of the lemonade powder and causes them to move rapidly. The bigger the kinetic energy is, more particles can come into contact with more water molecules and, thus, the lemonade powder dissolves faster.