On a recent outing, we came across something quite interesting that I don't want to keep from you. We were on a walk along a body of standing water when we saw some strange looking objects on the path ahead of us that looked like the fruit of a plant. Because of the water right next to us, I assumed that they might belong to some kind of aquatic plant, which later turned out to be the case. But the most striking thing was their shape, which immediately reminded me of a sharp metal objects, that you shouldn't step on with your foot. I immediately collected a few of these fruits to take a closer look at them.
As it turned out, it was the dried fruit of the water chestnut, an aquatic plant that was also native to Central Europe a long time ago. The fruits of the water chestnut become dry and extremely hard as soon as they are taken out of the water and their interesting shape immediately catches the eye. As I said, with their thorn-like and partly barbed appendages they reminded me of a caltrop, and in earlier times they served exactly the same purpose. These small, hard and pointed fruits were once used in Japan to slow down pursuing enemies or to defend fortified positions against foot soldiers. At that time, most Japanese still wore waraji sandals made of rice straw, and these sharp thorns of the water chestnut could easily pierce through and cause injuries to the soldier's feet, preventing them from advancing.
In today's world, the effect is probably not quite as spectacular; modern hard footwear should prevent injuries caused by a tennenbishi, a “natural caltrop”. This would probably require a caltrop made of metal, which could also make it through the sole of a shoe. But anyway, these things look quite fancy and I took a few of them home with me.
I have no idea what I might use them for one day. In Japan, people don't wear shoes indoors after all, and if you want or need to protect yourself from unannounced visitors, you might still find a field of application. You probably just need a little imagination.
I haven't done the ultimate test myself yet, but initial trials quickly showed that these little things can hurt quite a bit. In the past, most people probably had pretty thick skin under their feet and were quite tough. But if the Tennenbishi were already an effective defensive weapon back then and drilled into quite a few feet, they should still have the same effect on unprotected feet today.
As I said, they could be useful in your own home one day. I could drape my Tennenbishi around me while I sleep and if a ninja were to visit me at night, he might be in for an unexpected and serious surprise. But then I should definitely remember about them when I wake up and sneak through my house myself later. I don't want to start dancing in pain and end up cursing the person who came up with the stupid idea of caltrops around the house.
But anyway, now I have a few Tennenbishi in stock and I'm prepared for the worst-case scenario. Whatever that might look like....
