#1) The ocean has both heavy water and light water.

#2) Since various shellfish get oxygen from the water to make their shells, their shells reflect the percentage of heavy water in the ocean.

#3) Therefore, when the ice sheets are large, the ocean has a higher percentage of heavy water.

#4) Light water evaporates more easily than heavy water.

#5) Shells from dead ocean critters pile up on the ocean floor and we can study shells from thousands of years ago to find out how much ice there was then.

#6) Ice sheets are created from water that evaporated from the ocean.