To see what happens to brittle materials when they are chipped and knocked, get a bar of chocolate. Cooking chocolate is good because it is often very hard.

Place the chocolateon a large pastry board. The large board is to collect the chips we will make.

Now take a rolling pin or a small hammer and strike the chocolate sharply (you are 'knapping' it. Keep your fingers well away from the place you strike.

Examine the pieces that have broken away. You will find nearly all of tehm have sharp edges. It is a natural result of breaking brittle things.