Annual plants are dead or dying, and so are many animals which only live for a year or less. Life is in some ways easier for plants and animals where there are no great changes of warmth and light. The different kinds of plants and animals which live in countries with a cold frosty winter have adapted themselves to it in different ways. Living substance is generally killed by freezing, largely because ice crystals grow in it and break up its structure. In a properly organized society every factory and mine worker who wanted could get a month's work in the country in summer. Perhaps the most striking rhythmical changes in environment are those produced by tides. The plants and animals living in the tidal zone must be able to stand not only the violence of the waves, but drying up twice daily, not to mention frost and rain, unless they live in rockpools or burrow into sand or mud.