[Rohan_DHS]

To fly on a plane, they check your passport. On the passport, they check the stamps to make sure you are allowed to be at your starting point. Also, they make sure you are allowed to be at your destination. If they are a non citizen, they go through your passports more thoroughly. If something doesn't add up, as in Amadou's case, they even more thoroughly check through all of the traveler's documents, and either let them go, or prosecute them (depending on the circumstance, obviously) and possibly deport them.

ps. I don't know if it's too late or not, but if anyone wants to host a website to promote this, I am more than willing to help; just email me or pm me.