Here's a freebie. I though of this one for myself. Maybe not the first person to do so, but I was really tickled when it struck me.
Modern scientism says that because the Sun is so far away, its rays must come to the Earth in parallel. This is a major cornerstone of modern scientism; if it is not true, Eratosthenes' work was meaningless. But here's the thing... we can 100% prove that the Sun's rays DO NOT reach the Earth in parallel.
Did you ever build a pinhole camera/viewer in science class when you were in school? This is a simple device for viewing the Sun or a solar eclipse that can be made with a cardboard box, tinfoil, and tape. A pinhole is made in the tinfoil at one end of the box. This pinhole aperture is then pointed at the Sun to project an image at the back of the box, allowing you to view the Sun or eclipse indirectly. It's pretty cool! I made one when I was a kid. You can see images on Google.
Thing is, this device WOULD NOT WORK if the Sun's rays were coming in to the Earth in parallel. Parallel rays would simply come through the pinhole aperture and project a pinprick of light on the back of the box the same size as the pinhole aperture. It is because the Sun's rays are NOT reaching the Earth in parallel that this device works and projects an image that is bigger than the pinhole!
Man, it was such a rush to debunk a dogmatic claim of scientism that has deceived people for millennia!