What time is it . . . and how do you know? This fact- and photo-filled Smithsonian Penguin Young Reader will fill you in on how people first began measuring time and why knowing "when" is important. Time is when things happen and how long they take to happen--but how long is all that ? The history of telling time is the history of inventions and communication, from hour glasses to atomic clocks, from calendars to chronometers. Tick-Tock is a fun look at the many ways different people have tried to get in sync.