Here is one that I saw some place and thought it would be a good challenge.
Create a solid cylindrical shape with one end forming a 360° spiral. The surface of the spiral is such that a generally straight radial line is formed between a single center point and any point along the outside edge of the spiral. Refer to sections A-A and B-B for an example. There must be no central hole. The area below the spiral must be a complete cylindrical shape without gaps.
Overall dimensions and angles are unimportant as long as it generally looks like the example.
Note: Third angle projections.