Not every change can be turned back. Some changes make a brand new material that you can never turn back into what you started with. These are called irreversible changes.

Baking a cake is a perfect example. You mix flour, eggs, sugar, and butter into a runny batter. But once the oven bakes it, the batter becomes a soft, fluffy cake. You can never take the cake apart and get your eggs and flour back again.

Burning is another change you can't undo. When paper burns, it turns into ash, smoke, and gas. The ash is a new material, and there is no way to turn that ash back into a clean sheet of paper. The paper is gone for good.

Irreversible changes often make something that looks, feels, or smells different from before. A cracked egg cooked into an omelet, rusting metal, and a rotting apple are all changes that make new stuff and cannot be undone.

One clue that a change might be irreversible is heat, color, smell, or bubbles. Cooking food with heat usually makes a new material. When you see something change color or smell different, a new material may have been made.

Irreversible changes are useful even though we cannot undo them. Cooking makes food tasty and safe to eat, and burning wood keeps us warm. We just need to remember that once these changes happen, there is no going back.