Why
do we dream? Scientists aren’t completely sure, and they have diverse ideas.
Dreams might be a side effect of memory making. When you sleep, your brain
sorts through everything that happened
during the day, trying to link new experiences to old memories. As it connects
things, your brain turns them into a story, and you get a dream.
上面是摘自2021北京卷的一小段。其中的 happened 用得对吗?不应该改为happen 吗?