ELI5 Activity Trackers - how do they know you're asleep?

What are the creative ways people fool their FitBit? How does my activity tracker differentiate between steps and my tapping to a rhythm? Does it use GPS to calibrate? How does it know what stage of sleep I'm in?   ... We explain like i'm five!     Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: failtuna, fieldpeter, devilishgainz, srslylovesgames & groundhogcakeday To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]