brain

Think about your fifth-birthday party. Maybe your mom carried the cake. What did her face look like? If you have a hard time imagining the way she looked then rather than how she looks now, you're not alone.

The brain edits memories relentlessly, updating the past with new information. Scientists say that this isn't a question of having a bad memory. Instead, they think the brain updates memories to make them more relevant and useful now — even if they're not a true representation of the past.

Anthropologist Lynne Isbell was running through a glade in central Kenya in 1992 when something suddenly caused her to freeze in her tracks. "I stopped just in front of a cobra," she says. "It was raised with its hood spread out."

Isbell, who is at the University of California, Davis, says she has spent the past couple of decades trying to understand how she could have reacted before her conscious brain even had a chance to think — cobra!