Extruding entities is one way to create 3d solids. If you want to create some variety with your 3d solids you can use 'Extrude Random' to extrude closed polylines, spline, ellipses and circles. There are three way of creating extrusions based on two methods. With the first method you define a minimum and a maximum. For every extrusion a value will be picked randomly between these two values. This way it's very unlikely there are extrusions with exactly the same height because there are virtually endless possible values. To have a bit more control on values you can add an increment. If you have 5.0 as a minimum value, 7.0 as a maximum value and 0.5 as increment, the possible values are 5.0 5.5 6.0 6.5 and 7.0. For every extrusion the command will pick one of those values. If you want to have even more control on the possible values you can use the list method. That way you can define a list of possible values which have less relation with each other. For example you can create a list of values like 10.0 12.0 18.0 and 30.0. The command will randomly pick one of those values randomly for every extrusion.