$1 Pocket Microscope

This 3D printable pocket microscope magnifies up to 140x and can be built out of about $1 of parts!

Detailed instructions are here: https://www.instructables.com/id/The-1-Pocket-Microscope-Explore-the-World-Around-Y/

As a summary:
To assemble the microscope, you will need two glass bearing balls, 1/4" and 3/32" in diameter. You will need a 1" 4-40 screw. To hold a glass slide in place, you will need two jumbo paper clips. Finally, for illumination, you will need a CR2032 coin cell battery and a white LED.

Use the screw to form the hinge. Press the glass balls into their respective holes. Insert the battery into the pocket, and carefully slide the legs of the LED into the pocket as well, noting the direction and taking care to prevent shorts. Sandwich the specimen between two pieces of clear tape, and hold it in place using the paper clips, which slide into the rectangular holes in the microscope's bottom piece. Move the top piece up and down to focus. Look through the glass