Atmospheric Refraction
Atmospheric Refraction is the bending of light as it passes through layers of air at different temperatures.
Think of it this way
A straw in a glass of water looks bent at the surface. Water and air act like layers of cool and warm sky. Light slows down moving from air into water, just as it slows from warm air into cool air. That slowing makes the light path curve, shifting where the straw seems to be.
Explaining atmospheric refraction by grade level
When you look at a star, its light travels through the air to reach your eyes. The air has warm parts and cool parts. Light bends a little as it moves through each part. That is why stars seem to twinkle at night.