Antonyms let you double your vocabulary at once. If something 'expands' when it grows larger, which word means the opposite — to get smaller?

"Shrink" is the direct antonym of "expand," describing something becoming smaller in size, volume, or scope, exactly the opposite process of growing larger. Learning antonym pairs like this is an efficient way to absorb vocabulary, since understanding one word instantly clarifies the meaning of its opposite too.

"Expand" itself is the word being defined against, not its own opposite, so it can't be the answer. "Freeze" describes a change of physical state from liquid to solid and has nothing to do with size increasing or decreasing.

The expand/shrink pair shows up constantly in physical science: metals expand when heated and contract (shrink) when cooled, which is precisely why engineers build small gaps into bridges and railway tracks to accommodate this everyday behavior of materials.