Pop-Tarts is a brand of toaster pastries that the Kellogg Company introduced in 1964. Pop-Tarts have a sugary filling sealed inside two layers of thin, rectangular pastry crust. Most varieties are also frosted. Although sold pre-cooked, they are designed to be warmed inside a toaster or microwave oven. They are usually sold in pairs inside Mylar (previously foil) packages and do not require refrigeration. Pop-Tarts are produced in dozens of flavors, plus various one-time, seasonal, and "limited edition" flavors that appear for a short time.