.....and a metre is the distance travelled by light in a vacuum in 1/299,792,458 second. As you say, easy really!
... and a second is "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" (at a temperature of 0 K)
It gets simpler by the moment
(and the definition of a moment is ..........)
A work colleague explained to me what a "bit" was. I imagined it was something to do with binary storage in computer memory. No.
"two foot eight and a bit"
All was clear.