[交流] Funny English quiz in everyday（NO.9）

Previously
Q:What is easy to get into but hard to get out of?

A: Trouble.

Today’s Question
It is so weak, That a little wind can move it, It is so strong, That you cut it with a knife and leave no trace. What is it?

We will hold this column everyday, the one who write the right answer first can earned 1 BB.
If u missed it this time, please come on and come here early, then use your smart head to find the answer.
I will give u the right answer in the column tomorrow.
So good luck to u!