Like all batteries, eventually it gets depleted and needs to be replaced; this can be expected to happen after 5-10 years, depending on system usage. CMOS checksum error (and similar messages) or the system clock being reset are classic signs of the battery having been depleted. The BIOS settings contain several system-critical values.
In order to grow, yeast need glucose (C 6 H 12 O 6) or sucrose (table sugar, C 12 H 22 O 11), water (H 2 O) and warm temperatures. As yeast digest sugar, they turn into alcohol. In the process they give off carbon dioxide (CO 2) gas. The alcohol dissolves in the water and the gas escapes into the air. Yeast is used to make bread.
As the yeast feeds on the sugar, it produces carbon dioxide. With no place to go but up, this gas slowly fills the balloon. A very similar process happens as bread rises. Carbon dioxide from yeast fills thousands of balloonlike bubbles in the dough. Once the bread has baked, this is what gives the loaf its airy texture. Try the same experiment, but this time use about a tablespoon of baking powder instead of yeast, and leave out the sugar.