If it changes it isn't real.
If it doesn't change it can't be detected.
LuangPor's paraphrase of the W.W.I joke: Advice for Private Tommy Atkins.
In peace time:
If it moves, salute it.
If it doesn't move, whitewash it.
In war time:
If it moves, shoot it.
If it doesn't move, salute it.
Variations on this theme are quite common among English speaking people of LuangPow's generation.