Most kidney stones form when calcium and oxalate bind together in the body and crystallize. Those crystals can build up into a hard mass that gets stuck in your kidney or elsewhere in the urinary tract…until you pee it out. (Yeah, ouch.) Kale and other leafy greens are high in oxalate, so if you OD on them, you might get a stone. If you’re downing kale smoothies on the regular, mix it up a bit—there are many other delicious veggies to be juiced.