Freezing Point Depression
Freezing Point Depression is what happens when adding something like salt or sugar to water makes it freeze at a colder temperature.
Pure water in a pot freezes when the water molecules slow down and lock into a grid, like tiles on a floor. Salt crystals dropped into the pot break apart and scatter between the water molecules, blocking them from lining up. The salt particles act like pebbles thrown onto a tile floor — they get in the way and stop the tiles from fitting together. The water molecules have to slow down even more before they can freeze, so the freezing point drops lower.
Explaining freezing point depression by grade level
Think about plain water turning to ice in your freezer. Now stir in some salt and put it back. The salty water stays liquid longer because the salt gets in the way of ice forming. The water has to get even colder before it can freeze. Salt makes freezing harder.