(2) "One-leaf barrier" refers to the phenomenon that when light propagates along a straight line in a uniform medium, it encounters an opaque object and blocks the light, which belongs to the phenomenon that light propagates along a straight line, so it involves optical knowledge;

The moon in the water is the reflection of the moon in the sky in the water. The reflection is the phenomenon of plane mirror imaging, which is caused by the reflection of light. It is a virtual image of the moon, not a real moon, so the monkey fishing for the moon is "drawing water with a sieve".

So the answer is: ① sound source;

(2) the straight-line propagation of light;

③ The plane mirror becomes a virtual image.