Some proteins are soluble in aqueous solutions and some are not. Insoluble proteins can be a problem because the proteins can form large aggregates in solution which are difficult to purify, crystallize, and use in experiments. Compare...

Here's an abstract of a project from Science Buddies that asks you to experiment with bicycle gears and the circumference of the wheel to determine revolutions per minute.

Here is a challenging problem for anyone with an interest in geometry. This project requires background research to solve it, but it is an excellent illustration of visual thinking in mathematics.