Observer

looking is not free.

drag the lens toward the slits and watch the stripes dissolve

Particles fired one at a time at a pair of slits pile up into interference stripes, as though each one went through both. Bring a lens close enough to see which slit they take and the stripes melt into two plain heaps. Drag it away again and the stripes come back. Nothing changed but how closely anyone was looking -- and because the lens moves continuously, so does the pattern: certainty about the path and sharpness of the stripes trade off against each other exactly, which is what physicists mean by complementarity.

Concepts: quantum-mechanics, measurement.

An interactive you play with one thumb, on Wundr.