We all have open tubes of caulk which sit and dry out. So what is a person to do about it?
Using a flatblade screwdriver, scrape out about last inch of opening.
Using a q-tip, wipe inside clean.
Using a hot glue gun, fill inside with glue and make it slightly overflowing.
While glue is slightly cooled, but still soft, mushroom out the top. Let glue harden and you have an airtight seal to your caulk tube. No more waste.