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The only reason I can see for using a fork and knife to eat pizza would be if it's too hot to use your hands. And if you're impatient enough not to wait for your slice to cool, then I guess you could buy this Pizza Fork/Cutter, though you would have to be made fun of. A lot. The cutter is made with a fork built into a cutting wheel and lets you eat with just 1 utensil instead of 2. Fabulous. Now get a life.

Pizza Fork/Cutter [Kitchencontraptions]