ELI5 Carbonated Water - why does flat soda still taste like soda?

Why do soft drinks exclusively use carbon dioxide instead of gases like oxygen or nitrogen? Why does "flat" soda still have a distinct taste compared to regular tap water? Why does Perrier extract natural gas from its spring water only to pump it back in later? Does the carbonation in sparkling water affect your body’s hydration differently than drinking regular water? Why do smaller bottles of sparkling water go flat significantly faster than larger two-liter bottles?

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