Question

How does detergent change surface tension ?

Answer 1

Water is a polar molecule. The molecules are highly attracted to each other, thus creating high surface tension (that's why people rarely survive falls into water from high bridges).
Detergents have two ends: one is polar and the other end is hydrophobic (water hating). The polar end of the detergent can bond with the polar water molecules, reducing the water's surface tension. This is an advantage when washing clothes because this can allow the dirt and grease to be removed by water with detergent. Water alone cleans poorly because the water molecules are attracted too much to each other.