Sin

Photo by P'Tille
Using a slide rule used to be common, necessary knowledge among mathy types. In 1969, NASA and Bell Labs people still did most of their calculations with paper and a slide rule.
Nowadays you'd be hard pressed to find someone with any idea how a slide rule works, although using "slide rule precision" to mean "three digits" occasionally comes up. Well, there's a bunch of cheap ones on Ebay, and this illustrated slide rule course can let you experience history first hand.
A Polish analogue (!) computer used to solve differential equations, 1959.
The king of math vloggers, 3Blue1Brown, just released a video about the corona virus and exponential growth. It's probably a better Internet reaction to the epidemic than putting sliced apples in your bottom, which is what Facebook and Twitter are advising people to do.
This blog post on factorials of fractions helped clear up a tricky bit of math for me. Actually, pretty neat!
Logic can be used to solve problems, but it cannot suggest which problems to try. No one has ever formalized significance. To recognize what is significant you need a certain amount of experience, plus that elusive quality: intuition.