How do bike shifters work to change gears on a bicycle?

Bike shifters work by controlling the movement of the bike's derailleur, which moves the chain between different gears on the front and rear sprockets. When you shift gears using the shifter, it pulls a cable that moves the derailleur, causing the chain to move to a different gear ratio. This changes the resistance and speed of pedaling, allowing you to adjust to different terrain or riding conditions.