Well, often it involves a sense of mystery. You see someone and you don't know much about them, but you're intrigued. Body language is also important. If someone has an open and friendly body language, it can be more attractive. And sometimes it's just a gut feeling. You look at someone and there's this instant connection, like you've known them before in some other life. For instance, I once saw a woman at a fair, and she had this twinkle in her eye and a carefree laugh that made me have a crush at first sight.
One common element is physical appearance. For example, a really nice smile or beautiful eyes can catch someone's attention immediately. Another is the person's energy or aura. If they seem confident or have a certain charm, it can lead to that instant crush. Also, the setting can play a role. Meeting in a romantic place like a beach or a beautiful garden might make it more likely to have a crush at first sight.