Leftover pasta, with or without sauce, can be reheated in a skillet or casserole or used in a frittata. Cooked in a skillet with a little olive oil or butter over medium heat, the pieces turn crisp, brown, and chewy. Add a bit of water to guard against scorching or sticking. Serve the pasta for a quick meal with cooked garlicky greens, fried eggs, a sauce, or just a sprinkle of cheese.