Helmets are designed to reduce the impact to the skull in the case of a fall. There are different helmets for different types of riding. Road and mountain bike riders use light weight helmets designed to be aerodynamic and cool, using small holes to achieve both ends. BMX riders have less emphasis on weight and therefore heavier helmets are used which extend down to the nape of the neck, enclosing more of the head. High end time trial helmets are in the shape of a tear drop allowing for maximum aerodynamics.