system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] How do transistors work? What makes 3 chucks of silicon put together able to turn on and off?
gpt
Let's call the 3 "chunks" source, channel, and drain (in that order). The two ends (source/drain) have charge carriers available that allow current to flow through them if there's a voltage.  But the piece in-between, the channel, is depleted of charge carriers. If there are no charge carriers, there can't be current flow. Since that's what current flow is: the movement of charge carriers due to a voltage.   
    
So let's introduce a fourth piece: the gate electrode. It sits on top of the channel, but is separated from it by an insulator. And one more piece: the "bulk". It's kind of below the first three pieces.  
   
If you put a voltage on the gate electrode, it can pull charge carriers up from the bulk into the channel. At that point, it can conduct electrical current, and the transistor is "on".  Conversely, put the opposite voltage on the gate and it will push the carriers out of the channel and turn the transistor "off".