As an aside, standard marbles are almost exactly 2 Lego studs wide, which makes it easy to build marble runs using just standard Lego pieces. For instance, you

As an aside, standard marbles are almost exactly 2 Lego studs wide, which makes it easy to build marble runs using just standard Lego pieces. For instance, you can build a marble lifting tower for the start of a marble run that uses a 2-stud by 2-stud hole in a 4x4 (or 6x6 for strength) tower.