The primary contributor of skin color is the pigment known as melanin. The quantity of melanin in the skin varies based on genetic factors and is also dependent on the degree of sun exposure. Melanin is produced by specialized pigment cells known as melanocytes.
Average new tires used on cars typically start with 10/32 to 11/32 of original tread depth. Tires are considered worn out. In most States when they reach 2/32 of remaining tread depth. Ans 2 - Here in Canada (and I'm sure in many parts of the US ) we get a lot of rain.
Mechanical engineer working in automotive here. Car tire treads move water from the center of the tire to the outside edges. Many but not all motorcycle tire treads do the opposite, taking water from the outside edges and moving it towards the center. Sometimes the front and rear have opposite thread patterns.