Beat the egg whites until they start to get a bit thicker. Then add the sugar in two parts, beating between. Add the cream of tartar to help stabilize the foam. Add flavouring if desired. Continue beating until the mixture forms stiff peaks.
Note how the whisk beats air into the mixture, and that the air bubbles stay around as a foam.
Optional: carefully fold in food colouring to make swirly patterns.
Blob onto the tray and put in the preheated oven.
Cook until dry but not brown, about 1 hour. (With this cooking time it will be a little bit chewy in the centre. Yum!)
Allow to cool in the oven.
While they are cooking, do the foam molecule test to find out which components make the foamy texture - is it the protein of the egg whites or the sugar? Shake each in a small tube to find out. [protein in egg white makes foam, sugar does not]
Explanation: The foam texture of meringues is made by the protein of the egg whites, which surrounds the bubbles of air and stabilizes them. The protein molecules have different parts, some which like water ("hydrophilic") and some which do not ("hydrophobic"). The hydrophobic parts stick into the air bubbles (so only touch air) and the hydrophilic parts project into the water surrounding the bubbles. The protein molecules surrounding each air bubble stabilizes them so that they remain suspended in the mixture. When the meringue is baked the foam is hardened, to make a crunchy, airy dessert