This is because the top of the troposphere is normally very cold, causing ascending water vapour to freezeinto ice crystals that drift and fall, rather than continuing up into the stratosphere.
这是因为对流层的顶端通常极其寒冷，上升的水蒸汽凝结成飘落而下的冰晶，而没有继续上升进入平流层。
2
Put less food on your plate. Freeze your credit card into a block of ice.
放少一点的食物在你的盘子里，把你的信用卡冻结成一块冰。
3
By 2050, he said, millions of bored human beings would freeze themselves in order to emigrate into the future to find adventure.