The arbitrary cutoff size being to ensure continuity of the scientific consensus in popular awareness when I was a child isn’t a stupid rule.
Not even when a larger kuiper belt object is found.
Not even, when since mass is the primary means of estimating size until we fly a probe out there, we estimate a smaller but much with much more mass object to be larger and we debate a 10th planet yet again.
Other guy did a good job on the main points, but I’ll add something I saw in a study on a kind of bird in the US:
The birds realised cigarette butts had an antibacterial effect, and made efforts to collect and use cigarette butts in their nest building for eggs and chicks.
Learning and making use of novel materials.