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The problem with the Greggs cheese and onion pasty is that the bigger they make it, the hotter the inside will be, when you try and eat it.

So there's a size limit after which the cheese-like substance in the middle will hit the critical threshold and achieve fusion.

Which is brilliant if it happens and solves the world energy crisis. But not so great if I've just bitten into it and it's gone all over my tongue...