Question

Some life forms move so slowly that their movement cannot be detected by normal observation. The best way to determine any movement is by placing a mark at some point, and observing any change in position from it. Why is this true?

Answer 1

Not sure the precise concept of "normal observation", but I assume that is observed by "eyes".

Eye observation is basically macroscopic, but when you use a mark, which can be regarded as a point of mass, then it goes to microscopic.

Mark is a reference point which you can compare the relative position change, but with your eyes, first you cannot notice microscopic changes, second the eyes cannot precisely set a stable reference point.

Answer 2

The mark is a reference point.