Yeast
Yeast is a tiny living thing that eats sugar and makes gas, which is why bread dough rises.
Yeast cells are tiny round dots floating in a bowl of sugar water. Each dot eats the sugar around it and releases small gas bubbles. The bubbles rise up and push the dough above them higher and higher. A flat lump of dough grows into a tall, puffy dome as the gas fills it from inside.
Explaining yeast by grade level
Mix yeast with warm water and sugar in a bag. The bag puffs up because yeast makes gas. Yeast is alive, just too small to see. It eats sugar and turns it into bubbles and a sour smell.
Projects that explore yeast
You place 5 grams of dried baker's yeast in a sealed plastic bag, and inside a second perforated bag you crush 150 grams of grapes to release the juice. As the juice mixes with the yeast, gas fills the bag and foam appears over the next hour. Testing the gas with a glowing splint confirms it: carbon dioxide snuffs it out. The pH drops as the juice turns more acidic.
Yeast cells eat sugar and release carbon dioxide gas. Billions of cells working together produce enough gas to inflate a sealed bag. That same gas is what makes bread dough rise.
Yeast is a tiny living thing that eats sugar and makes gas. You add one gram of yeast to containers of orange, apple, and cranberry-grape juice. The juice with more sugar produces more gas.
Yeast breaks down sugars and releases carbon dioxide, but not all sugars work equally well. You dissolve different carbohydrates in water, adjust the pH to 6.5, add yeast, and warm the mixture to 37.5 degrees Celsius. Sucrose and glucose produce lots of gas. Others like galactose and lactose produce almost none. The results show that yeast is picky about which sugars it can use.