Instant fish were a species of African killifish (Nothobranchius sp.) that laid eggs in the mud at the start of the dry season. The eggs lay dormant (embryonic diapause) until the rainy season, when the water would cause them to hatch and be able to survive in full rainy season rivers and lakes, rather than the dry creeks of the dry season. Wham-O had the idea of selling chunks of a mud substitute with the eggs inside it. Once this was taken home and put in a suitable container, water would be added and the eggs would hatch.