The story
The foamy texture and size of the resulting mass suggest a large amount of toothpaste, as might be imagined to be used by a large creature like an elephant.
Etymology from Wiktionary, CC BY-SA 4.0.
elephant's toothpaste
Today it means: A foamy substance caused by the rapid decomposition of hydrogen peroxide using potassium iodide as a catalyst; the outcome of a popular educational experiment in chemistry. Full definition →