What is the water string used for on a trumpet?

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That's a water SPRING, not a string.
That's a replacement spring for your water (spit) key. In case you don't know what a water key is, think of how you empty your trumpet of spit--there's a key near the bell on one of the tubes that you press and blow out the spit. That spring is used to fix your water key should your water key spring break.