Character mascots for Kellogg's Rice Krispies (Rice Bubbles, down under) cereal.
In 1928, Kellogg's introduced Rice Krispies on grocery store shelves, with advertising that distinguished the cereal as one that "crackles in cream." Starting in 1932, boxes of the product depicted a bowl with onomatopoeic words emanating from the cereal: snap, crackle, and pop, and at the same time, radio jingles repeated the message:
Listen to the fairy song of health, the merry chorus sung by
Kellogg's® Rice Krispies® as they merrily snap, crackle, and pop in a
bowl of milk. If you've never heard food talking, now is your chance.
While there's a plausible scientific reason why the cereal makes noise (the addition of liquid breaks the fragile walls of the toasted rice), the company instead decided to advertise the noise as the invention of magical characters. Illustrator Vernon Grant first drew a gnome with a baker's hat and dubbed him Snap. Snap began appearing on Rice Krispies boxes in 1933, and was soon joined by two others that Grant naturally named Crackle and Pop.
In 1939, Kellogg's created archenemies for the trio (Soggy, Mushy, and Toughy) in the animated short subject, "Breakfast Pals." Despite the archetypal battle waged over the the edenic domain of a family breakfast table, the villains were easily vanquished in less than a minute. Strangely enough, they were vanquished not by Rice Krispies but by large quantities of syrup and pancakes deftly wielded by Snap, Crackle, and Pop.
(See for yourself: https://youtu.be/vl0p2-_YPeM?t=17 )
Sources:
"The Tale – Snap! Crackle! & Pop!® story." Rice Krispies official web site. http://www.ricekrispies.com/The_Tale.aspx (September 16, 2008)