Why does leather get hardened after tanning ?

Give reasons for the following observations: Leather gets hardened after tanning.

Give reasons for the following : (a) Leather gets hardened after tannig ( b) Fe Cl_3 is preferred over KCl in case of a cut leading to bleeding .

Give reasons for the following : (a) Leather gets hardened after tannig ( b) Fe Cl_3 is preferred over KCl in case of a cut leading to bleeding .

Why does the picture get distored?

Why does rifle bullet get heated on striking a target ?