People can actually do two or more things at once, such as walk and talk, or chew gum and read a map; but, like computers, what we can’t do is focus on two things at once. Our attention bounces back and forth. This is fine for computers, but it has serious repercussions in humans. Two airliners are cleared to land on the same runway. A patient is given the wrong medicine. A toddler is left unattended in the bathtub. What all these potential tragedies share is that people are trying to do too many things at once and forget to do something they should do.
