Do you ever eat a fresh spinach salad and find your teeth feeling strange and chalky? You're not alone. "Spinach teeth" is caused by the high quantities of oxalic acid found in spinach. The oxalate crystals leak out from spinach as you chew, and these crystals coat the teeth, which results in that gritty feeling.

And it doesn't just come from spinach! Rhubarb, beets, kale, and chocolate are high in oxalic acid as well. Beyond that strange dental sensation, this acid increase your risk of oxalate stones in the kidneys, so people with a predisposition for such issues should steer clear of eating large amounts of spinach.

The oxalic acid contained in spinach can also prevent your body from absorbing iron. To improve iron absorption, spinach should be eaten with foods that contain vitamin C. So the next time you're chowing down on some cooked spinach, be sure to squeeze a little fresh lemon on top!