Not easily washed away

Not only stones show a hard crust
against all kinds of an attack.
Near water covers are a must:
what water takes will not come back.

Persistent tides with their erosion
form all the coastlines every day.
And even granite knows corrosion,
and becomes smooth when wet like clay.

Words, especially when written,
always remain hard and dry.
And all those who have once been bitten
will remember and be shy.