Have you ever known someone for a while and it's fun at first, but then things seem kind of off. Then you start to notice a fib here and there. Then, over time, you hear things that contradict what you heard earlier. You may hear a story you told them coming out of their mouth like it happened to them, or a girlfriend story turns into a sister story.
Finally you hear them tell one you know is absolutely false, and what's worse, you have no idea why they would even bother lying about such a thing.
That's when you know they're a pathological liar. They don't lie to cover up their screwups; they don't lie to make themselves look better; they don't lie for self-gain. They simply lie because they don't think anything they say really matters - they're just words. "Hey look - the sky is pink." is just something to say because they felt like it.
Yep, looks like we've got one in the Oval Office.