This toy is designed for people learning about velocity profiles in fluids. The slider represents the progression of time and different slats move at different rates to create velocity profiles as might be found in the Navier-Stokes equation. The slats can be rearranged to create different profiles, and can be used to show two sided and one sided non-slip boundary conditions.

For a simple demonstration I recommend the setup for pipe flow, which is requires:

1x Base Piece, 1x Slider Piece, 2x Every Slat Piece

With this setup you can make a lot of different profiles, but feel free to download and make different slats to fit whatever profile you want. All models were made in OpenSCAD.