It's dark. The road you once knew is now under construction. It's become unpredictable, throwing up new, constantly changing surfaces and obstacles. You do your best to apprehend them, but it's a losing battle.
A thud reverberates from your wheels, through the frame and into your forearms.
It happens again, only this time it's worse and the thud gives way to an aggressive, enduring hiss.
Both of your tubes have burst. You've packed one spare, it's 06.30 in the morning and you're a long way from where you need to be.
You dismount, you put your bike on your shoulder and you begin the long, frustrated walk to the office.
From now on, I won't be leaving home without at least 2 inner-tubes, even if the ride is short.
Every day's a school day.