One of my favorite records to research is the census. They are little time capsules. One of the lessons I learned early on is that you have to prove what you find. Sometimes even in a record.
An example is this 1900 Federal census taken in Dale County, Alabama. While it shows my Great Great grandparents, Wilburn and Mollie Snell, as being married 9 years, which is true, I can tell you those are not her 8 children. I would not know this until I had also documented Wilburn's first marriage. In fact, only the last 4 are from this marriage to my ancestor, Mollie.
And there is my Great grandfather, Jamie Wesley, as a 6 year old. I barely remember him, mostly his funeral, when I was a small child. I always thought of him as old, but there he is, a child himself.