Eggshells have a porous structure. That is, they allow air to pass through. As eggs age, they absorb air and form an air pocket. In general, you can test the freshness of an egg by placing it in a cup of water. If the egg floats, it means it is old and has a large air pocket, and you should avoid eating it. If the egg remains on the bottom, it is usually safe to eat. You can smell an egg before eating it to ensure that it is fresh. If it smells sour, you should avoid it.