It's easy to tell if a skeleton is lying to you because you can see right through them.
Skeletal Deception
Categories
Explanation
Experiment
Explain Bear
Listen up, genius! Skeletons don't have skin, so you can literally see right through them. The joke is using "see through" in a literal sense, not just in terms of detecting a lie. Get it now, or do I need to draw you a diagram?