Why Can't They Allow Someone's Toe to Be Offside?

My understanding of the offside law, why it was brought in, was to stop a player, a goal-langing. Basically, you could stick a big guy right up in the bench, just leave him there, and then you lump a ball up to him. That is offside. One toe, a three centimeters. It's ridiculous, and it's killing football. You've seen a perfectly good goal disenact. Now, our game's between full – it doesn't mean much to us. But for Christ's sake, if that's an FA Cup final or a Champions League final, you're going to disallow a goal because someone's toe is offside?