Small bomb thrown by hand

My absence from these pages has been primarily because I hurt my neck and sitting at the computer has been rather uncomfortable. In truth it has also been in part because nothing of interest has happened to me or indeed anywhere near me. However, I was awoken from my torpor today when the centre of town was suddenly sealed off by the police and several buildings evacuated. It turned out that someone had donated a hand grenade to Oxfam. As you do.

A better man than me could probably make a joke out of the fact that since shops are only accepting contactless payment at the moment, anyone who bought the grenade from Oxfam wouldn't need a PIN. However, until that better man comes along, we are left with the one about the mathematician being trained to use grenades. He was told to pull the pin and then count down from three before throwing it. It blew up when he reached minus six.