Run in place very fast for a minute. Do you notice how hard you are breathing? What you are breathing is oxygen? You need oxygen to live. That's why you can only hold your breath for a certain amount of time.
You can't see oxygen. It's invisible. It is a gas. A gas is matter that has no shape or size of its own. Gases have no colour.
Gases are all around you. You can feel gas when the wind blows. The wind is moving air. Air is many gases mixed together.
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