Take the test: 1. How many kilometres do you ride each year (a)? 2. What approximately is your average speed (b)? 3. Your average hours / year riding? (c = a / b) 4. How many years have you had your current helmet (d)? 5. The number of hours in your current helmet? (e = c * d) For me, the answer comes out to: a) 15,000 b) 50km/h c) 300 d) 2 e) 600 Imagine a t-shirt that you wore for that many hours without washing. Now, aren't you inspired to wash your helmet lining?