10% of aspartame breaks down into formaldehyde, a known carcinogen. It isn't a coincidence that aspartame and formaldehyde are linked to the same types of cancer. ELI5? Hmmm... the chemical mechanisms are beyond a 5 year old. But, when you raise 10,000 rats and feed half aspartame but not the other half, certain cancers are more prevalent in the aspartame group AND their offspring.