Although vinegar hardens the shells, the flesh will soften up if there is any gap in the shell, making the conker useless. Every conkerer disagrees with every other on how best to produce an invincible nut. As such disputes are an essential part of the sport, we leave the question with the totally contradictory answers given above a. I was intrigued by all the dialogue about conkers, and I gather that they are chestnuts, somehow attached on a string. I suspect it doesn't involve consumption.
Conkers are the hard fruit of the horse chestnut tree.
These are collected in autumn, removed from their spiky casing and left to mature. A hole is then drilled in the conker, and a string threaded through. One player dangles their conker by the string, holding it steady, while the opponent swings their conker on its string and attempts to strike the hanging conker. Players take it in turns to do this until one conker is so damaged that it is dislodged from its string. The winner is obviously the player with the intact conker.