Base 4 Theme Day

Today, Base 4 have had their final theme day of the year. The children dressed up as early medieval Britain and spent the day as invaders and settlers. The children had to create their village, living like those in 600 AD by making jewellery, weaving clothes, and grinding flour. They also explored riddles, which was loved by the settles during this period in time. Can you solve this one?

When I am alive I do not speak.
Anyone who wants to takes me captive and cuts off my head.
They bite my bare body.
I do no harm to anyone unless they cut me first.
Then I soon make them cry.