Be Prepared.
Baden Powell recommended it. The strange hill-billy goat in “Hoodwinked” sang a cute song about it. As a former boy scout, someone who appreciates the value of having an appropriate horn available when it’s needed, (and of course, a rider of a homemade recumbent bike) I thoroughly endorse it too. So here’s the contents of the tool kit I currently lug about on my bike these days.

I also carry a mobile phone and spare batteries for my lights, but they’re not in the tool kit. Is there anything you think I’ve forgotten?
But wait! There’s more. Visit the Fat Cyclist who will explain what you should carry in order to attain popularity. It will be a life altering experience.
Condoms. You forgot condoms.
Dean, I guess I do need something to put tools into.