If you ever see "rhubarb" on a menu, chances are you will wonder what it is unless you are a gardener. Rhubarb is a leafy green plant with a bright red stem, and has been used as a laxative for centuries (starting in China). It is now grown worldwide. Rhubarb is usually found in dishes with strawberries, because red and red totally go together, dahling.
No, you aren't eating the nice, green leaves of the plant. You're eating the stems. Just the stems.
The leaves of rhubarb are poisonous. They contain high levels of oxalic acid, a corrosive and nephrotoxic substance found in naturally high amounts in rhubarb. Spinach, often touted as one of the healthiest plants around, contains enough of this stuff to be worrisome (and would make a great entry on its own). The poison is usually used as a cleaner, especially for removing rust. Symptoms of rhubarb poisoning include weakness, burning in the mouth, difficulty breathing, coma, and, umm, death. We warned you about the glue; this is even worse.
Cooking does not get the toxins out of rhubarb. Ordinarily, one would have to eat a lot of rhubarb (5 kg of leaves) to die from it. When rhubarb was used as a cheap vegetable in WWI, people ate enough of the leaves to die of *gasp* RHUBARB POISONING. There is still a small amount of the toxin in the stem, but it would take eating rhubarb like a crack addict to make one throw up. (Disclaimer: We are not responsible for the consequences of rhubarb being dealt in alleyways. K. Thanks.)
Remember that rhubarb is poisonous next time you chow down on strawberry-rhubarb pie. You have just eaten one of the fugu-ish members of the plant world.