Walnuts are one hard nut to crack, so people use a hammer, rock or mallet to crack them open and get at the delicious meat inside, and when they find a good nutcracker they stick with it.
A villager from Shaanxi province, China thought the "mallet" his friend gave him in the early 80s was the perfect walnut cracking device, until he saw it on a government flyer- and found out it was actually a hand grenade.
Finding out his mallet was actually a Chinese Type 67 defensive hand grenade came as quite a shock to the man, who quickly gave it to the local police so he wouldn't get in trouble.
But he would have been even more shocked had it gone off in his hand while he was bashing walnuts!
-Via Popular Mechanics