The Key Concept concerning Time: Simultaneity
Two things either happen at the same time or they don’t? Right??
If time is the same everywhere for all observers, then it is a simple matter to determine whether two things happen at the same time - each observer just looks at his watch.
Einstein’s response: maybe, maybe not.. I don’t know it means until we define what we mean by things (we’ll call them events from now on) happening at the same time.