Your brain is (probably) not shrinking as you age!
Contrarily, as a child, I was terrified when an adult told me, "If you do not use your brain, it will rot in your skull!" Thing is, he was (essentially) correct. On the other hand, the "trashy" science fiction I was reading instead of doing my homework is pretty good brain exercise; on the other other hand, passing Trig on my first attempt would've been better.
3 months ago