Teresa Gardner, age 8, of Greensburg, Kentucky, for her question:

Why does the frost kill only some of the plants?

All the growing green leaves are alive and all of them are made of boxy little plant cells that are too small for our eyes to see. The little cells are filled with sap and other watery plant juices. And in cold weather, watery liquids tend fo freeze and become solid ice. When the frost freezes the sap inside a leaf, the tiny cells are torn and broken by jagged bits of ice. The leaf cannot mend its wounds. When the tender leaves of a plant are bitten by frost, they wither and die.

The frost kills all the leaves that are delicate and paper thin. But some plants have leaves that are covered with thick skins. The needles of a Christmas tree are leaves that are specially made to withstand the frost. The leaves of evergreens have thick skins to keep out the cold. Their thick, slippery skins can shed the snow and stop the frost from changing their watery sap into jagged spikes of ice.