I think that seed on the left is a horse chestnut. We call them conkers here. In the olden days, children would thread them on a piece of string and have conker fights. In turn, each person would let their conker dangle on its string, and the other would try and strike it with their conker. There were lots of techniques to the striking, and soaking of conkers in things like vinegar, purportedly to make them very hard and unbreakable (the object of the fight). I seem to remember they were banned in my school as too dangerous 😱