Nozzle caps, or nozzle socks, are great for tubes of caulking and glue bottles. They have a humorous look, but they do work better than the traditional nail or wooden plug. I’ve used them to store latex caulk, silicone caulk, and a marine adhesive called Sikaflex. For capping silicone caulk, Lee Valley suggests to leave a small gap at the tip, which fills with a little of the caulk to form a plug. This doesn’t stick to the nozzle caps. So far they have kept everything fresh.
[Nozzle condoms also go over anything else with precious fluids you like to keep wet: markers and color pens, for instance. -- KK]