When a melted chocolate is kept in a refrigerator it solidifies on cooling. However when it will be kept at room temperature, it will absorb heat and melt again. Hence, it is a reversible process..

The changes that can be undone are reversible change. Growing of moustache over a boy's face can be undone by shaving it. So it is a reversible change.

Making stars by cutting it out from a paper isn't a reversible change because once you cut out a star you can't join it back to make the same paper. Since we started making the stars out of a plane big sheet of paper and ended up with the stars, we cannot join the star back to get the complete sheet of paper back.

When a metal is heated, it expands and when it is cooled, it contracts. So, heating and cooling both brings about a change. Hence, both the statements are correct.

Not all changes around us are desired. There are many changes which can harm us. For example, global warming changes the temperature of our planet. However, if the temperature of the earth increases beyond a certain limit, it will become inhabitable for mankind.

A paper boat can be made from a paper and then by unfolding it, it can be converted back to the same paper. Thus this is a reversible change. Whereas when a plane drawn on a paper is cut out, then it cannot be put back at the same position, this change is permanent. Hence this change is irreversible.

Sand castles can be broken back to sand, so it is a reversible change. On the other hand, making egg omelette, cutting of trees and burning petrol are irreversible as they cannot be brought back to their original state.