One day in summer, Brenda sneaked into an auction and stole two big diamonds. After returning home, she put the diamonds in water and then put them in the refrigerator for freezing. Because diamonds are transparent and colorless, once hidden in ice, they are not easy to find when the police come to search.

The detective came to Brenda's house the next day.

"Hand over the diamonds you stole! Although there is no evidence. But when I first saw you, I knew it was you. "

"If you doubt me, why not just search?" Brenda said.

"How about a coke? It's very hot today. "

Brenda took the ice from the refrigerator. Put four in each cup. Put those who hide diamonds in her cup. Even if the ice melts, the diamond will not be found by half of Cole.

The detective picked up his glass and drank it. Unconsciously glanced at Brenda's cup.

"Excuse me, can we change the cups?"

"Do you think I put poison in your cup?

"Not poison, but the diamond is in your cup.

Finally, the detective recovered the diamond.

How did the detective know that the diamond was in Brenda's cup before the ice melted?

The answer is that ice cubes should float on the water, while diamonds will sink because they are heavier than water.