I got picked on a lot in elementary school for having yellowy teeth (among other things). A friend's mom suggested I rinse twice a day with STRAIGHT hydrogen peroxide. I now know it's used as a mouthwash but is diluted and certainly not done as a daily practice unless medically necessary, but what did I know as an insecure 6th grader?
Luckily, my dad caught me doing that about a week into my "new regimen" and told me to knock it off before I destroyed my teeth. In retrospect, my friend's family always had a lot of cavities and I distinctively remember her refusing ice water, ice cream, etc, because it "hurt her teeth," so there's that. I just Googled it and apparently now idiots on TikTok are recommending people do this.