Mac and cheese becomes creamy mainly due to a combination of factors: using a good amount of fat (like butter or cream), making a proper roux (a mixture of flour and fat) to thicken the sauce, using whole milk or heavy cream, and carefully melting the cheese into the sauce over low heat to prevent clumping or separation. Cheese block shreds are preferable to pre-shredded for better melting, and certain additives like sodium citrate (found in processed cheeses like Velveeta) can enhance creaminess by keeping the cheese sauce smooth and velvety. Gradual heating, low temperatures, and patience during melting help maintain the creamy texture without curdling or becoming grainy. Adding ingredients like cream cheese or eggs can further enrich and smooth the sauce. Avoiding high heat and boiling ensures the sauce doesn't break or dry out, while ingredients like evaporated milk or half and half can also improve richness and creaminess.