Sometimes,there is too much water in a cloud.It gets too heavy and falls down.That's rain.
翻译为：有时,太多的水在云层里,导致太重了 ,就落了下来 ,这就是雨
解析：
too much water：太多的水
too heavy：太重；太沉重了 （这里你打错了个词）
fall down
跌倒；失败；倒塌