Sea-Floor Spreading
Sea-floor spreading is the slow movement of ocean floor away from cracks where hot rock rises up.
Think of it this way
A baker rolls pizza dough from the center outward. New dough forms at the middle and pushes old dough to the sides. The center stays active while the outer edges grow further apart. This is how new rock forms at mid-ocean ridges and spreads the sea floor wide.
Explaining sea-floor spreading by grade level
Deep under the sea, the ground has cracks. Hot rock comes up through the cracks. The new rock pushes the old rock to each side. A shoebox model shows how the ground moves apart.