How to visualize joint uniform distribution

Lets say you have X and Y, where the joint density function for X and Y is uniform over the region defined by 0<=x<=y<=L, where L is some positive constant.

The question asks for the expected value of the squares of X and Y.

I am having trouble visualizing what such a distribution would look like. Apparently it is triangular shaped... but I do not see it. Can anyone help?

$$E[X^2]=L^2/6$$
$$E[Y^2]=L^2/2$$