A Trojan horse looks like a gift but hides an attack. Greek soldiers hid inside a wooden horse.
After ten years of war, the Greeks left a giant wooden horse outside Troy and pretended to sail away. The Trojans pulled it inside the walls. That night, soldiers hidden inside opened the gates.
That 'free' ranch-land offer with a 40-page contract? Check it for a Trojan horse.
In IT, a Trojan is malware hidden in a normal-looking file. Don't open strange attachments.
Practice it in Skepsis