For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
I wish we could test this out with only simple apparatus. Unfortunately the common people do not have access to satellites or nonstop bullet trains.
Nobody has access to the speed of light (or anything near it).
We can and do. GPS satellites need to be regularly calibrated to Earth clock signals or they’ll start to drift their calibration by meters per day.