Shelf life / Expiration reminder

Porblem
It is very unobvious to see the expiration date on the pack of the toast. Consumers usually ignore the expiration date because it is just a small number that printed on the pack. Consumers should be more careful of the expiration date in order to prevent the toast from becoming moldy.

Concept
Users stick a transparent sticker on the pack of the toast, the special chemical material, acid alkali will combine with oxygen. Days by days, the color will change in order to remind users that the toast is going to out of expiration date.