Christmas crackers owe their crack to a compound called silver fulminate. Fulminates contain the fulminate ion, CNO–, the instability of which leads to fulminate salts being friction sensitive explosives. Silver fulminate is at the finer end of this sensitivity; it can detonate with very little provocation, and is in fact so sensitive that, if prepared in a quantity more than a few milligrams, it can easily self-detonate under its own weight.

So if you're planning on making your own crackers this Christmas, beware of the bang!