Q: Why are ghosts so bad at lying?
A: Because you can see right through them!

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: