Scientists can identify planets in distant star systems that could be conducive to life. They can create ships that launch people into space and put them on the moon. But there's a ton of common, everyday things that science does not understand.
For instance, scientists can't explain why we cry when we're sad. They can explain why babies cry, or why we tear up when something gets in our eyes. But not specifically why we cry when sad. Or we also don't know why the heck anesthesia makes people pass out. We just know it's effective. In the video above, Mental Floss discusses 19 common things we've probably all experienced that scientists cannot explain, although they have a few guesses.
Good luck going into your next surgery knowing scientists have no idea why you're passing out on that strange mixture of chemicals!
(h/t Mental Floss)