Some things were already normal when you came into the world, and will likely still be normal when you finally depart it. It's only natural to assume that those have always been around -- that they're an ageless part of the landscape, like the mountains, the clouds, or Cloris Leachman. Not so. Everything people do or make had to be done or made for the first time -- and for some things, that first time happened shockingly close to your birth. Staples of human experience (or at least your human experience) would surprise/horrorize/gross out 99% of humans who have ever lived. Try showing these to your elderly great-grandfather, and see how he reacts.
Source: Encyclopedia Britannica
Source: Christ and Pop Culture