Question: why does a pencil look bigger when it is in water?

Oliver Gordon answered on 5 Mar 2019:

So, light travels at different speeds through different materials. This is because different materials have a different “refractive index”. If you were to shine a light at a different material, at the edge of them you have reflections (the light bounces off, aka a mirror) and refractions (the light keeps travelling into the new material). The refractions cause the light to not go straight on, and the angle that light refracts depends on its refractive index. (This is partly how we design glasses lenses for people with poor eyesight!).

Now, your brain assumes that the light beam is going in a straight line, but because the light is refracted it reduces in angle, so has less distance to the bottom of the water. Things then appear closer to you then they are. This is why water also seems shallower than it is, tricking some people and making them jump into very deep lakes without fear of drowning.