面团在冰箱里放了一夜后变硬了。

The dough ___stiffens after being left in the fridge overnight.

加入明胶可以让果冻变得更结实。

Adding gelatin ___stiffens the jelly to make it firmer.

当他听到坏消息时，他的表情突然变得僵硬。

His expression ___stiffens when he hears the bad news.