Answer:
Defends your noggin from the hard pavement
What breaks in the water but never on land?
What time of day, when written in a capital letters, is the same forwards, backwards and upside down?
I start in little but I end in full, you'll find me in half and complete.
Used to keep prisoners silent and to make temporary repairs on cars.
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.