Some products solve a problem so directly and so effectively that they need very little explanation. The Herco Spitball is one of them. 

At Wind Instrument Proshop we carry both sizes of Herco Spitballs — small bore for trumpet, cornet and French horn, and large bore for trombone, baritone, and euphonium. Every jar ships across Canada

HOW SPITBALLS WORK
Insert a Spitball approximately six inches into the lead pipe using a pencil or similar object. Blow hard to send the ball through the horn. Discard the used Spitball. The foam disc travels the full length of the instrument's bore, absorbing moisture and lifting residue from interior surfaces that no cleaning brush can reach — the bends, the valves, the curves deep inside the leadpipe where buildup accumulates over months and years of playing. 
Spitballs absorb excess moisture and remove loose residue from the inside of the horn, helping to clean and sanitise the instrument. Used once a week as part of a regular maintenance routine, they prevent the gradual accumulation of debris that compromises tone, intonation and valve response. Used monthly on a well-maintained instrument, one Spitball comes out clean. Used on a neglected instrument for the first time, the results can be genuinely shocking — 

Herco Spitballs are the most cost-effective brass instrument maintenance product available. A jar costs less than a single valve oil purchase and protects an instrument worth hundreds or thousands of dollars. For school programs managing large numbers of student brass instruments, regular Spitball use is one of the simplest ways to extend the playing life of the fleet and reduce repair costs over time. Both sizes ship across Canada.