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What use is a cast iron frying pan full of rocks?

When I bake bread I put the pan full of rocks on the bottom of my oven when I turn it on. By the time the temp is where I want it that frying pan and the rocks in it are very hot. That's when I slide in the proofed dough and toss in a cupful of water to create a burst of steam.
So why the rocks? To increase the surface area, thus creating a bigger burst of steam.
Plus it's cool to have a frying pan full of rocks sitting around in my kitchen because every now and then a visitor will ask "What's the frying pan full of rocks for?" And I can say "Ballast."