This reminds me of when my kids were little. The first time my son threw something off his high chair, I told him "no", and he never did it again.
Then I had twin girls. The first time one of them threw something off their high chair, I told her "no". Well, her twin sister starts laughing and copies her sister. I tell her no, and they both start throwing stuff off their high chairs.
I then had to take them both out of the high chairs and stop eating. Of course, this happened repeatedly because the twins would just laugh at each other. Double trouble with my twins.