Watch the two light streams move in opposite directions inside a transparent 3D prism. Decide which sheet looks closer, then keep looking for a depth flip that swaps the front and back motion.
Music: "Bushwick Tarantella" by Kevin MacLeod (incompetech.com), licensed CC BY 3.0 US. Excerpt trimmed to 30 seconds and mixed with this video.
Source: https://commons.wikimedia.org/wiki/File:Bushwick_Tarantella_(MacLeod,_Kevin)_(ISRC_USUAN1300002).oga
License: https://creativecommons.org/licenses/by/3.0/us/deed.en