In school physics, we teach about the diffraction of waves - both of light and sound and water. This is the effect as waves hit an aperture that they spread out; it's what allows us to hear round corners, but not see round corners. As a general rule, the best diffraction effect is observed when the wavelength of the wave is approximately the same as the width of the aperture.
Later, in Years 12 and 13, we teach that even electrons show diffraction patterns when accelerated through certain crystal lattices.
So, the question I usually set my students is: why do we humans not diffract if we run through an open doorway?
Don't Panic! I will give the answer (if needed), but it is much more interesting to hear other people's.