If you look up the definitions today, a second is defined as the duration of 9,192,631,770 periods of a caesium-133 atom's… vibration (I suppose?) while the meter is defined as the distance travelled by light in 1 / 299,792,458 seconds.

These definitions were definitely never taught to me in school, growing up. I guess I never asked enough questions as a kid, and just accepted second to be 1 / 60 of a minute, which is 1 / 60 of an hour, which is 1 / 24 of a day, while meter is pretty much the length stated by every ruler I've ever owned.

What if a kid is to ask me these questions today, though? Or if my future kid asks it? How and why is a second a second, or a meter is a meter? How do I explain these things in a way a kid can relate to, without going into what's caesium, or what's caesium-133, or why caesium, or why 9,192,631,770?

The meter is definitely something I'll grapple with too. I accepted the meter for what it is in day-to-day life before I learnt the speed of light in school, so the concept of defining the meter according to the distance travelled by light feels extremely circular-logic-ish in my head.

What's the accepted layman, non-pedantic definitions for SI units (not just second and meter) commonly used in day-to-day life, in terms which is relatable in day-to-day contexts?