In The Early 20th Century City-Dwelling Mothers Would Put Their Babies In What Curious Contraption?
Answer: Window Cages
Human history in general, but especially the history of child rearing, is filled with interesting (and often dangerous ways) in which people adopt what seems to be common-sense advice and apply it in strange ways.
In the 1920s, a trend started taking hold in urban centers in the United States and Europe in which families would affix a large metal cage to an open window (similarly to how we, in the modern era, stick window-mounted air conditioners out of open windows) expressly to put babies and toddlers into.
What compelled thousands of mothers to stick their babies in window cages? The belief that sunshine and fresh air was critical to the growth and development of their babies. The idea, popularized by one Dr. Luther Emmett Holt in his widely read 1894 book on child rearing and health The Care and Feeding of Children, was that fresh air was required to renew and purify the blood and exposing children to the elements (within reason) helped fortify their immune systems.
Now, while Holt’s understanding of the matter might not have been scientifically rigorous by modern standards, assuming the cages were safely installed and the baby/toddler was at no risk of falling, we can’t really find a big fault with the devices no matter how curious they might seem to the modern reader. After all, sitting in the sunshine, watching the hustle and bustle of city life, and getting a big dose of fresh air (or, in terms of city air quality, at least fresher than the air inside the apartment) hardly seems like an unhealthy way to spend a little time in the afternoon.