Emergency tyre boot - set of 3 - TB-2 - from Park Tool USA

Rarely needed, but excellent if you do.

These are cheap, last forever in your tool kit and weigh nothing. More resistant to sharps than either bank notes or Duck tape, if you ever have to use one of these, replace the affected tyre as soon as possible.
If you replace tyres before they're too far gone, you're unlikely to need this, but you'll almost certainly one day be riding with someone who does. When that happens, you will:
a) be regarded an a bike-wrenching deity,
b) be plied with enough beer and coffee to make the original purchase worth while, and most of all
c) get your group moving again instead of suffering repeated punctures.

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