This appears to be the very latest game for kids.
For whatever reason some cars now have a new kind of number plate, it is black
( kuro) with yellow ( kiiro) lettering.
It can be played by pedestrians but seems to be played largely from in the car.
What you do is keep your eyes peeled for a car with this kind of plate, if you see one, you screech ‘ KUROKI’ in my ear, and then clap twice and ‘ frame’ the plate with your fingers.
I am not making this up.