Why are some kind of packets-soups boiled up in lukewarm water, while normally just stirred into already boiling water?
Some ingredients do not dissolve well in hot water - the starch swells and thickens, forming lumps that may have raw powder in them and are nasty. They need to be added to cold or lukewarm water and heated after they are dissolved. Other ingredients, most notably pasta, will partially dissolve in cold water making a thick gloppy soup. But if you add them to hot water they will "seal" (in a way) and stay together as they cook.