Light and shadows
A shadow forms when an opaque object blocks light from a source, and moving the object closer to the source makes the shadow bigger.
A worked example
Hold a book close to a flashlight and its shadow on the wall is large and blurry. Move the same book farther from the flashlight and closer to the wall, and the shadow shrinks and sharpens.
The mistake people actually make
A shadow is not a copy of the object's colour or detail, only its outline. Confusing luminous objects, which make their own light like the Sun, with objects that simply reflect light, like the Moon, is the other common slip.