I assumed that rand would take a range as an argument. Something like rand(10..20), generating a random number between 10 and 20. Seems like you'd do this fairly often when working with random numbers and therefore, included. However, it doesn't work. But the solution is almost as easy.
10 + rand(11) #=> produces a random number between 10 and 20
Since rand starts at 0 (like array indexes), we need to add an extra 1 to get what we're expecting.