How are lactic and alcoholic fermentation similar?

Lactic and alcholic happens when there is no oxygen.

So, how did it happen?
Recall that in glycolysis of cellular respiration, there is a glucose (6 Carbons) molecule. That glucose molecule is going to be broken down into two pyruvates (3 Carbons). There is no oxygen (also know as anaerobic respiration) so pyruvate is going to be converted into lactate or alcoholic fermentation depending on it's accessibility. I highly suggest you to watch this video if you don't get what I'm talking about:

http://www.bozemanscience.com/cellular-respiration/

Explanation:

When you heat the air it expand its volume, but its mass remain the same, so the density of the hot air balloon is smaller than the density of the air outside it, so the hot air balloon can float.

In the case of the helium balloon, there's no source of energy, but the helium is naturally lighter than the atmospheric air, so its density is naturally smaller than the air, and the helium balloon have a smaller density than the air too, so it can float.