In this demo illustrates the concept of marginal and conditional probabilities. In the left, you have joint density $f_{XY}(x,y)$. Use the selection bellow to change it. Bellow it and to the right of it yu have two sliders where you adjust the value for the conditional in $X$ and $Y$. You can move the sliders to change the value and observe the changes in the conditional and marginal densities in the plots bellow. Two checkboxes are there to show the planes for the conditional values of $X$ and $Y$.