Cashew shells contain urushiol, the same toxic oil found in poison ivy, which is why cashews are always sold shelled.
Why it is true
The cashew grows attached to the bottom of a fruit called the cashew apple. The nut's shell contains urushiol, a substance that can cause severe allergic skin reactions. Therefore, cashews are always processed and roasted at high temperatures to neutralize this toxin before consumption.
Source: Smithsonian
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