[fferen]

I see. I don't know of a great answer, but it comes down to the constant speed of light. The simplest clock is two parallel mirrors with light bouncing back and forth. If someone is moving, then the light seems to take longer to bounce between them, so their clock appears slower. Wiki has a a good explanation:

https://en.wikipedia.org/wiki/Time_dilation#Simple_inference

In the hypothetical party scenario, you could demonstrate with your hands or some objects as mirrors.

Not the most satisfying answer, but if relativity was super intuitive, I probably wouldn't have written this book :)