We publish a short physics video, read the numbers YouTube gives us, and the next video changes because of them. These are the clearest lessons so far. We're a small channel, so we trust rates (how many viewers keep watching, how many like) over raw counts of comments or subscribers, which are too small to mean much yet.
Same idea, new picture: yes. Same picture: no
Our first video of light rays bouncing inside a cup reached about 25,000 views in two days, many times our usual. The next one, rays inside a bowl playing Carol of the Bells, reached about 15,000 in a little over a day.
Then we published a third that changed only the ending and the song: same bowl, same look. It got 134 views in its first day. The people who did see it liked it (70% chose to keep watching instead of swiping away, as high as our best), but the feed barely showed it. The video after that kept the same idea and changed the picture to an ellipse, and it was back to about 2,800 views after roughly a day.
The lesson: repeating a format is fine, repeating an image isn't. Every video now has to look different in its very first second.
Long endings lose people
YouTube shows what share of viewers is still watching at each second of a video. In the cup video the curve stayed high almost to the end, then fell from about 72% to about 32% during a two-second explosion at the very end. People had heard the song, and they swiped away.
We cut the ending to under a second, and the next video showed no drop at the end. The audience for that one was small, so we're still watching it, but every video since uses the short ending.
Viewers notice bounces that look steered
Under one of our spike videos the only two comments were about the bounces: one asked why the ball takes a different route every time it lands on the first ball, the other said it was rigged. Each bounce was physically correct. So we measured the whole path, and the ball was drifting sideways two to three times more than in other bounce videos we measured, because its starting point moved a little for every ball to spread them out.
Nobody could see the starting point move, but they could feel it. The lesson: a bounce can be correct and still look steered. Since then we measure sideways drift before a spike video goes out.
People really do play along
Under the Carol of the Bells video, 7 of the first 11 comments named the song, several of them adding the round where they got it: "I got it on 100". One even wrote its Ukrainian name, Shchedryk. In a video all you can do is comment. That's why this site exists: here you can play. Try Name This Song, or guess which ball escapes.