Because oceanic lithosphere cools and thickens as it ages, it subsides. The mixture is brought to a boil then simmered until it thickens. Palm sugar is produced by boiling collected sap until it thickens. Estrogen thickens the endometrium of the uterus. This paste thickens the mixture, although it is boiled for an additional few minutes. In hypertrophic cardiomyopathy the heart muscle enlarges and thickens. Connective tissue contains collagen, which is converted into gelatin that thickens the liquid. The nail often thickens and becomes brittle, turns brown or black. The vaginal mucosa thickens and the vaginal pH becomes acidic again. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus. The head also thickens, the olfactory organ enlarges, and their teeth are lost. After the shear is removed the ketchup thickens to its original viscosity. During a woman's menstrual cycle, the endometrium thickens in preparation for potential pregnancy. The skin thickens, embedded scales develop and the body divides into segments. The baking also thickens the texture more than the reaction alone.