When egg whites are beaten, some of the hydrogen bonds in the proteins break, causing the proteins to unfold ("denature") and to aggregate non-specifically. This change in structure leads to the stiff consistency required for meringues. The use of a copper bowl, or the addition of cream of tartar is required to additionally denature the proteins to create the firm peaks, otherwise the whites will not be firm. Plastic bowls, wet or greasy bowls will likely result in the meringue mix being prevented from becoming peaky. Wiping the bowl with a wedge of lemon to remove any traces of grease can often help the process.