孩子作业,帮忙看看是什么答案 what's it? it's so weak,that a little wind can move it,it is so strong

孩子作业,帮忙看看是什么答案 what's it? it's so weak,that a little wind can move it,it is so strong
that a knife can't leave and trace on it.
what's it?
it's-------
它很弱小,一点点风就能吹动.它很强大,尖刀也留不下痕迹.它是什么?