Keep your number of takeoffs equal to your number of landings (idea)

It is important to note that for most people, this invariant is unmaintainable if one ever wants to fly. Thus most people cannot fly if they wish to follow these wise words.

However, some people are born while on a plane, making it impossible for them to maintain the invariant on the ground. Such people would do best to ignore this saying, or they will surely crash at a very early age.