What do you think is the hardest food to cut open with knives?

Yesterday I decided to test my cutco knives on some of the hardest food items I could think of. I bought Watermelon, squash, and pineapple and my knives cut through them all like butter it was amazing!.

I also bought a coconut thinking I could use the dull end of my butcher knife to smash it open but no cigar.

So think of things harder than a pineapple, watermelon, or squash, but softer than a coconut.

Whatchu got? :D
What do you think is the hardest food to cut open with knives?
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