Surfactants
Surfactants are molecules in soap that grab onto both water and oil, letting you wash greasy things clean.
Think of it this way
Soap molecules act like tiny two-headed helpers in your dishwater. One head of each molecule grabs onto grease on your pan. The other head grabs onto the water around it. When you rinse, the water pulls the grease away because the soap molecules hold both sides at once.
Explaining surfactants by grade level
When you add dish soap to a bowl of milk with food coloring, the colors race away from the soap. That happens because soap breaks the pull that holds the milk's surface together. Without that pull, the colors swirl and spread into new shapes. The soap changes how the liquid holds itself.