number one and number two as euphemisms for urinate/defecate. rationale for which is which?

After years of never knowing which is which, I finally looked it up and it seems number one is firmly taking a pee, while number two is taking a poo.

This seems quite arbitrary so I am wondering the rationale for it? (I spoke French as a child to my parents, rather than English, so I don't have an early familiarity to fall back on - crude English slang is what I used with other kids).

Is it that we normally tend to pee way more often than we take a crap so that ends up as number 1? But you could equally say that defecating is way more complicated and problematic if you aren't near proper facilities. Especially for men who find it easy to pee messily in emergencies. So that would make it a candidate for #1.

Or is it just a convention without basis?

Also, are non-North Americans English speakers (UK, Ireland, Australia, NZ...) also familiar with it?

Solution 1:

The euphemisms #1 and #2 are most needed by parents of small children. Small children usually learn to control urination first.