New word: Caltrop

	So. I was kicking myself for not taking a photo of or writing down the name of those weird horny bat things I saw at the Berkeley Bowl. I googled around for a while, led astray by their being displayed with the citrus fruit. Well, they are indeed a fruit, but are called a nut, and that's how I found them eventually.
They are the fruit of a water plant called Trapa bicornis, being the two-horned variety of Trapa natans, otherwise known as water caltrop. They are also called bull nut, buffalo nut or devil nut. They are sometimes called water chestnuts (so Manisha wasn't really wrong in the previous comments), but are no relation to the true water chestnut (Eleocharis dulcis), which looks like this:(I ate many of these while traveling by bus in southern China. They are much sweeter fresh than canned, and are kind of fiddly to peel, so they made a good diversion for whiling away the hours on the road.) Here's what the fruit of Trapa natans looks like. The two-horned variety is called batnut or devilnut for obvious reasons. The four-pronged variety goes by caltrop nut:
Searching for the word "caltrop," I found that it's a kind of weapon that's been used throughout history. It's a sharply pronged device that is structured so that no matter how it lands, there is always a prong sticking up straight.
Here are two caltrops I found online. The one on the left is a modern version that can be used to puncture car tires (being hollow, with that hole at the juncture, means even a self-sealing tire can't prevent the air from escaping). The one on the right was dug up at the Jamestown settlement (read about it in "New Discoveries at Jamestown" at The Gutenberg Project).
Which brings me to my next discovery, that the much-dreaded burr I've known as the goatshead burr, devil's head burr or tire-puncture burr is also known as a caltrop burr. Native to Africa and Asia, this wicked plant is now spreading in the U.S., possibly imported originally in sheep's wool. It is the bane of bicyclists for obvious reasons, and also of dog owners. Sophie has gotten them in her feet many times; I know to check her pads for them as soon as she starts limping.
(Once we visited a new off-leash dog park the city had opened next to the pound, which besides being a horrid location with all the pathetic strays in pens looking on, turned out to be a small, bare, dirt lot surrounded by chainlink and filled with caltrop vines. The dogs took one lap around and came back limping on all four feet, their pads packed with vicious burrs. We never went back.)
	But back to the water caltrop. In Chinese it is called ling jiao and is considered good luck because of its bat shape (bats are lucky). Also, the character ling is pronounced the same (with a rising tone) as the character for intelligence. Lucky brain food!

The caltrop must be cooked to destroy the toxins it contains in its raw state. You can roast it, boil it or steam it, and reportedly it tastes very much like actual chestnuts. Naturally there are many Chinese dishes that feature it, and also a traditional Italian risotto. If you google "caltrop recipe" you will find many ways to use it. (And now I'm wishing I'd bought some to try!)

If you can read German, here is a super-comprehensive page on the caltrop both as weapon and plant. If you can't read German, you can still get the gist by looking at the photos.

So now you know more than you ever thought you would about this crazy nut, caltrop!