Moral is the passage of time: the moral of the hourglass is the passage of time, mainly because the hourglass is a timing tool, and the process of sand flowing from one end to the other symbolizes the passage of time. The sand in the hourglass slides down one after another, just like one after another over time. Once it has passed, it can never be restored to its original state.
It symbolizes love, friendship and happiness: because the process of sand flowing in the hourglass is also the process of recalling the beautiful fragments between friends. The process of sand flowing in the hourglass needs patient and meticulous observation, just like love and friendship, we need to manage and maintain it with our heart.