|Double-slit experiment, with the interference pattern on the right|
and the two vertical slits in the center as S2. CC BY-SA 3.0,
via Wikimedia Commons
Richard Feynman came up with the following analogy: Imagine you're shooting at a wall, but between you and the wall is a sheet with two vertical slits. The logical assumption would be that the bullets would hit the wall in two corresponding vertical rows--but that isn't what happens with light. Instead, when you shine light at two vertical slits, it builds up in an interference pattern (several bright and dark bands), which is a characteristic of waves, not particles.
When you think about light, do you tend to think of it as a wave or a particle?
-----The Golden Eagle