When Ron was a little kid, he was a bit gullible. Here are some of the little 'jokes' Fred and George played on him.
Fred: Hey, Ron, we were at Divination today and we learned something really brilliant.
Ron: What?
George: Professor Trelawney told us a neat way to tell when you are gonna die.
Ron How?
Fred: Well, you've got to figure out when your great-grandfather died and then subtract the number of freckles you have from that number.
*Ron runs to the bathroom mirror*
Ron (panicked): I was due to die five days ago!
Fred and George: The spiders are gonna get you, Ron!
George: You know I've heard neon orange polka-dots are very 'in' this season.
Fred: Really? I thought it was neon green…
Ron: Does Madam Malkin sell those?
Ron: How do they decide what House you get into when you go to Hogwarts?
Fred: It's a test.
George: Yeah, you have to…
Fred: Wrestle a troll.
George: Yeah, and if you win, you get to be in Gryffindor.
Fred: And if you lose you go to Slytherin.
George: And if you chicken out you go to Hufflepuff.
Fred: And if you tie you go to Ravenclaw.
George: And if you die you go to the cemetery.
Ron: Do they take exchange students at Durmstrang?