The spent outer casing of the edible chestnut. I picked this up on my walk this morning- the chestnuts in the UK are very, very small given our cold and wet climate. The fruit forms in late summer and is ready to harvest in late autumn when the spiky enclosure (pictured) bursts open and falls to the ground. The chestnuts can then be retrieved by carefully pulling them out of their needle-encrusted home. I have collected chestnuts near where I live but the pain ain't worth it. Even picking this one up from the ground hurt like hell and my fingers are now slightly swollen. It is useful to know where food comes from and the effort that goes into getting it to our plates. I can only imagine that machinery does the job of picking and collecting the chestnuts. Or the fingers of the pickers have been so frequently impaled they no longer feel pain. One has to marvel at nature: by making it quasi-impossible for predators to 'break in', the chestnuts have the best chance of germinating into saplings the following year.
The analogy being here that by protecting oneself to the hilt, one is less likely to come to harm. Although I would argue that it is wise to be a little bit nuts occasionally.