Hint S3 - K'NEX Skid plate/Eye centre
This special part is a smooth dome that fits onto the end of any classic rod, as shown in the photo.
One use of the part is to place it underneath the back end of a K'NEX vehicle which is driven by two motors. It can then be used as a "skid plate" when the vehicles turns a corner.
A further use of the part may be found in all the K'NEX Motobots sets.
Here the part is attached onto a K'NEX rod, and then fitted on top of a K'NEX motorycle hub with coloured tyre.
This creates an effective large "eye" for the model.