That is defined as, let's say I have one independent, standard normally distributed variable. I'll call that x1, and I square it. And then I have another independent, standard normally distributed variable x2, and I square it. So you can imagine, both of these guys have distributions like this, and they're independent. So to get to sample q2, you essentially sample x1 from this distribution, square that value, sample x2 from the same distribution essentially, square that value, and then add the two, and you're going to get q2. This over here, we would write, so this is q1. q2 here is a chi-squared distributed random variable with two degrees of freedom.