How to prevent leftover paint from drying out in the can?

Even so. Would you seriously open a 5-year-old bucket of paint and start painting?

Just a quick note on the topic of "old paint"... Yesterday I opened an old leftover bucket of silicate interior paint from 17.5.2017. So almost 7 years old! There was a kind of oil or thick (clear) liquid settled on top of the paint, but underneath the paint was buttery soft and completely intact. I stirred a few spoonfuls, including the liquid, in a small bowl and touched up some spots on the wall. Perfect!! Today everything has dried and you can't see any transitions.