Mars used the slogan “Melts in your mouth, not in your hand” for M&M’s. The candy’s hard sugar shell was designed to protect the chocolate center from melting quickly in the hand.
M&M’s were introduced in the United States in 1941 by Forrest Mars Sr. and William M. F. “Bruce” Murrie. The product was inspired partly by chocolate pellets with a hard coating that Mars had seen being eaten by soldiers during the Spanish Civil War. The coating made the candy practical for storage and transport.
The slogan describes the product’s intended advantage, not an absolute physical rule. M&M’s can melt when exposed to sufficient heat, and the chocolate inside can still soften. The brand name refers to the surnames Mars and Murrie, the two original business partners.