Leftover vegetables went bad in two days

Why

Lata's rule matches most of what she has seen — covered food does last longer, so air looks guilty. Nikhil's counter-example is the move that cracks it open: dry biscuits sit in air for months without spoiling, so air by itself cannot be what does the damage. What air carries is the real culprit — invisible bacteria and mould spores drifting in it. They land on the food and grow, and they need moisture and warmth to do so. Damp cooked vegetables give them everything; a dry biscuit gives them almost no water, which is why it merely goes soft rather than rotting. Test: leave half a chapati dry and sprinkle the other half with water, both covered — only the wet half grows mould. So it is the microorganisms arriving with the air, not the air, and that is why we cool food, dry it, or seal it.