Our bizarre mystery seed from last week was a water caltrop, Trapa bicornis.
The water caltrop (also called horn nut, etc.) is a nondescript little freshwater plant. It is originally from Asia, where it has been cultivated for over 3000 years. Inside the fruit above is one large seed, which can be eaten after it has been boiled. (Some sources say the seed is toxic if not boiled, others say that raw seeds may harbor parasites. Should be cooked in any case. )
If we had known what it was when we got it, we could have put it in our pond or fish tank. Now it is probably too old to grow.
I wonder if the horns help it catch in other plants when it disperses in the water?
Have you ever seen a water caltrop? Do you know this plant by any other names?