You are looking at a three year old tube of silicone which is still soft and uncured. The secret? A seal of cling wrap (also known as Saran Wrap). Simply tear off a small piece, place it over the threaded tube end, and screw the cap back down. The seal this makes prevents moisture from getting to, and hardening, the silicone.

Permatex “RTV” gasket silicone is a must for the engine spares. It is often used in place of paper gaskets, and works well at high temperatures.