How do the yellow petals of a dandelion transform into those white puffy seed parachutes?

The yellow petals of a dandelion do not transform into white puffy seed parachutes. The white puffy seed parachutes are actually modified sepals, not petals. The petals are the showy, brightly colored parts of the flower that attract pollinators. Once pollination has occurred, the petals wilt and fall off. The sepals, which are small, leaf-like structures at the base of the flower, then enlarge and turn white. They become attached to the seeds, forming the parachutes that allow the seeds to be dispersed by the wind.