Why is this the answer?

The answer is yeast because this clever riddle is describing a tiny, magical helper that makes our bread soft and fluffy. Let's look at the first part: "What must be in the oven yet can not be baked?" Yeast is a living, single-celled fungus that a baker adds to the bread dough. The baker puts the entire batch of dough into the oven so the bread can get baked and become delicious. The yeast is right there inside the dough, so it is in the oven, but the yeast's main job of making the dough light is done before the baking even starts. We never say we are just "baking the yeast." Its whole purpose is to make the bread rise before it cooks. The next part says it "Grows in the heat yet shuns the light of day." Yeast is very particular about where it works best. When you mix it with warm water and a little sugar, it wakes up and starts to eat the sugar. The warmth helps it grow and multiply quickly. A baker will always cover the dough and keep it in a cozy, dark spot, away from the "light of day." This quiet, comfortable place is perfect for the yeast to do its important work of puffing up the dough without getting a hard crust on top. The third clue is super clever: "What sinks in water but rises with air?" When you first mix the small, dry yeast granules into water, they are a little heavy and sink to the bottom of the bowl. But once the yeast is awake and eating the sugar, it starts to make lots of tiny bubbles of carbon dioxide gas. Think of these as little burps that get trapped inside the sticky dough. These bubbles are a kind of air, and they push the dough up, making it rise and grow big. This "air" makes the whole dough float higher in the bowl. Finally, "Looks like skin but is fine as hair?" When you buy yeast at the store, it often comes as tiny, fine granules, like a powder or a thin strand of hair. After the dough has risen and is ready to bake, the top surface often forms a smooth, thin layer that is like a "skin" over the puffy bread. All these special clues together perfectly point to the hardworking yeast.