646.

It's not just good manners—the "rule of reciprocity" suggests that we're programmed to want to help someone who's helped us. It probably developed because, to keep society working smoothly, people need to help each other out. Stores (and some frenemies) like to use this against you, offering freebies in hopes that you'll spend some cash.

647.

In a famous 1950s experiment, college students were asked to point out which of three lines was the same length as a fourth. When they heard others (who were in on the experiment) choose an answer that was clearly wrong, the participants followed their lead and gave that same wrong answer.

648. According to the name-letter effect, people are more likely to be attracted to someone who shares their initials. For example, a man named Robert will tend to be attracted someone named Rachel or Roberta.

649. The opposite of paranoia is pronoia. A person suffering from pronoia feels that people or entities around them are plotting to do them good.

650. "Transabled" people are people who cut off their own limbs to become disabled. They do this to make themselves feel more comfortable in their own bodies.