Yes, it’s from a book called “Uncle John’s Bathroom Reader” and that book was resting on a closed toilet seat in a nation that only kind of uses the metric system in certain situations… however, you’d be surprised how much you can learn after a single visit to any local fast-food joint!
Or my favourite ratio, 1l of water is 1kg and it’s close to that for most food grade liquids.
It is great when cooking, your kitchen scale (which you should own!) works as an easy way to measure liquids as well. Need 500ml of water? Just measure 500g and you are done.
While technically it is only true for very specific water and temperatures at sea level, for everday cooking it is close enough with almost any liquid.