How do scientists know...?

Question

how did scientists come up with the theory that if you be perfectly still a tyranosaurus rex will not sense your presense

Answer 1

Many predators have visual systems that key on motion. That's why you often see deer, rabbits, and other prey animals standing completely motionless. It's certainly plausible that tyrannosaurs used a similar strategy, though I'm not sure there is any scientific basis for this. The idea seems to have originated in the book "Jurassic Park" and  was repeated in the movie.

Answer 2

In a silly movie they know. Confusion between amphibians and dinosaurs, I think.