A greasy plate is rinsed with (a) pure water and (b) detergent solution. Explain the difference in cleaning using surface tension and contact angle. Step 1: (a) Pure water has high T (0.072 N/m) and a large contact angle on grease → it beads up, cannot penetrate under the grease film, and the grease stays stuck. Step 2: (b) Detergent lowers T (~0.03 N/m) and the contact angle → water spreads and creeps under the grease (roll-up), peeling it off. Step 3: Detergent tails embed in the loosened grease, forming micelles that disperse in water and rinse away. Answer: Detergent wins by lowering surface tension and contact angle (wetting) plus emulsifying grease into micelles. Pure water beads and fails on both counts.