Not too long ago, I enjoyed my favorite steakhouse side, creamed spinach, with a reliable hunk of aged meat at the House of Prime Rib. I hadn't had the dish in a while, and when I took my first bite I instantly remembered what it felt like to have "spinach teeth" — a gritty, uncomfortable sensation that one experiences after eating spinach. Why, I wondered, does spinach leave your teeth feeling rough?For starters, spinach grows best in sandy soil, so the grittiness may very well come from actual sand if it's not been thoroughly washed. But on top of this, the plant's leaves contain a high amount of oxalic acid crystals, which spread onto the teeth during the chewing process, resulting in a chalky sensation in the mouth. Spinach teeth may be enhanced when the vegetable is eaten with iron-rich substances like milk, which explains why the phenomenon is so detectable with creamed spinach.