"Teacher melted several substances during the lesson by heating them," Fred added. "But while he was melting them he gave a boy the piece of ice to hold in his hand. When he asked the boy for the ice he found it was all gone.""Of course," said Norah, "the heat of the hand melted the ice.""Quite true, but would the other substances have melted in the same way?""Butter would have melted in the hand, but the wax, sugar, sulphur, and lead would not have melted.""Why not?" asked Fred again.
"These substances require more heat than ice requires before they melt," said Norah.
"Now, I want you to think about some of the things you have seen melted," said Fred.