Whether a foreign operative, marketer, or troll, anyone today can manipulate others into believing and latching onto what they want by disseminating misinformation. But the human mind, scientists contend, is built for belief. When you picture or hear of something, you assume it's true. Our ancestors evolved in an environment too dangerous to question themselves every time they thought they saw a lion or second-guess every story from a tribe member. Credence and impressionability, and the folding of hearsay and conviction into memory networks, are not flaws—they're efficiencies in building a cohesive cosmology. But they leave us vulnerable to predatory rumormongers.