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Although seems silly, I think this is a great question, given the context. If you were to put the same force on a nail, as on the knife while chopping the tomato, clearly the force would be more concentrated (higher pressure)and hence the tomato would be easily ruptured. However, to 'chop' a tomato we need forces acting along a line, which is why we need an extended object like a knife. So in short, it's a 'geometric requirement of forces' (for the lack of better words)for 'chopping' to work. Hope that made sense.