You're right to be curious! It's a bit of an optical illusion. While the Moon's shape does change during its phases, the entire disc is always there, even during the new moon. Here's why you might see the "round outline" of the moon during a crescent phase:

* Earthshine: The Earth reflects sunlight back onto the Moon, faintly illuminating the unlit portion of the lunar surface. This soft glow, known as "earthshine," can make the entire disc visible, even though most of it is not directly illuminated by the Sun.

* Atmospheric Scattering: The Earth's atmosphere can scatter sunlight in a way that makes the entire lunar disc appear faintly visible, even when it's mostly dark. This is especially noticeable when the Moon is low on the horizon.

* Contrast and Perception: Our eyes are designed to pick up subtle differences in light and shadow. Even when the Moon is mostly dark, our brains may perceive the outline of the full disk due to the contrast between the brightly lit crescent and the surrounding darkness.

Important Note: You won't ever actually see a fully round moon during the new moon phase. This is because the illuminated portion of the Moon is facing directly away from Earth at that time. However, you can still see hints of the full disc thanks to these factors.