Q: Why are ghosts so bad at lying? A: Because you can see right through them!\nWhat do you think is the punchline of the above joke?
Explanation: To “see through” someone means to understand them, what they really mean, to understand the truth. You can “see through” someone who is telling a lie; that is, you can tell that they are lying, not telling the truth.
Of course you can see through a ghost because ghosts are transparent.
Do you play Minecraft? Here is how to make a ghost (transparent player) in Minecraft: