The small K'NEX 25mm wheel black is provided in numerous K'NEX sets. This wheel comes in two different versions, as shown in the photo.
The wheel shown at the top of the photo is the "Closed-centre" version of the wheel. This can be attached to the end of any classic K'NEX rod, where it will spin freely.
The wheel shown at the bottom of the photo is the "Open-centred" version of the wheel. This can be pushed onto any classic K'NEX rod. However, it will not spin on the rod, but will turn with the rod, because it is quite a tight fit.
The Open-centred wheel can also be used effectively as a wheel for Micro K'NEX models.
This is achieved by fitting a Micro K'NEX spacer to a Micro K'NEX rod, and then fitting the wheel outside the spacer, as shown in the photo. The wheel will then run freely.
The open-centred wheel can also be fixed more firmly to a classic K'NEX rod using a Tan interlocking clip (see Hint W6).
The photo shows one of the wheels being used as a handle to turn a rod attached to a pulley.