How to tell an egg is fresh or not is a tough question in daily life. The egg freshness tag is an intuitive solution to know the freshness of egg by simply refer to its colour. Green means fresh and red means bad. Inspired by chemical indicator, the pH of solutions can be expressed in different colour scale, what if the freshness of the egg can be expressed in different colours as well? The egg freshness tag is a one-direction carbon dioxide chemical indicator filter which changes its colour according to the amount of CO2 emits from the egg.