Spin off on my scarred for life thread- door lock q?

[SnuggleBuggles]

If you have a lock on your bedroom door (or if your parents had them), how did you explain the concept to your dc? Do we tell ds that if the door is locked that mom and dad don't want to be disturbed? Do we give any reason or explanation? I have a feeling ds will want to know why there is a lock all of a sudden.

TIA!
Beth