Mmm, Poisonous Fruit
Apparently the reason you can’t get cashews in the shell is that the shell is poisonous. And the nut doesn’t even grow inside the shell.
Makes me think of castor beans. Castor oil has many uses in industry, medicine and food. You eat it all the time in most blended vegitable oils. However, the outer hull of the castor bean contains a posison called ricin, which is one of the most lethal poisons known. 150 μg of ricin (about as much material as there is in one LSD tab) could kill a grown man. My thesis advisor was once poisoned with ricin. He was working on castor beans in the lab and spilled a little (non-leathal amount) on his hand. Apparently it felt like his whole arm was on fire. Not something I care to experience.
On an interesting side note: when one considers that most of the most poisonous compounds are actually proteins, their lethality becomes even more staggering. For botulinum toxin, a 150 kDa protein, a lethal dose works out to be like 1/10th of a molecule of the toxin per cell in your body. Assuming the average number of cells in a body is ~10 trillion. Though if the toxin concentrated in neurons, we may be dealing with a number more like ~10 molecules per neuron.