Watch the striped ball travel along the straight center guide, then look slightly to one side. The ball never leaves that line, but motion inside it can shift the path you see and create a strong curveball optical illusion.
Music: "Peaceful Desolation" 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:Peaceful_Desolation_(Kevin_MacLeod)_(ISRC_USUAN1200017).oga
License: https://creativecommons.org/licenses/by/3.0/us/deed.en