Families Would Use The Same Bath Water
Water heaters weren’t exactly common housing components during the 1800s, so it actually took a lot of work to heat up water for baths. As such, most families simply bathed one at the same time, one right after the other, rather than go through the hassle of heating water and filling the tub numerous times. The oldest always went first starting with the grandparents, so you can imagine how gross that water must have been after a good half-dozen people used it.
Fun fact: the phrase “Don’t throw the baby out with the bathwater” stemmed from this common practice.