A half life is a probability defining when 50% of an element is expected to decay.  Its possible that all of the element decayed or that none of the element decayed, but this is extremely unlikely.  Think about it this way, if you flip a coin two times you can get 100% heads, 50% heads, or 0% heads.  But as you continue to flip coins, say you did it 10,000 times, you will get about 5,000 heads and 5,000 tails because probability tends to balance large numbers out to represtent their expected proportion.

The second half life is when 50% of what is left over decayed. Mathematically speaking, it would take a very long time for all of an elemental material to decay. You'd go from billions of atoms of material to two atoms, then one half life later one decays leaving you with one atom.  And that last atom would have a 50% chance of decaying by the next half life.

Quantum physics introduced probability into physics, and probabilities are what we are dealing with here.

Why is the sky blue?

Because half life is awesome.

Somebody had to.

Gases in the atmosphere.

Blue light scatters more than red/green light in the atmosphere.

How can there be a vaccine for HIV when a vaccine is the strengthening of the immune system, and HIV is the weakening of the immune system?

There isn't a vaccine for HIV virus. There is a partially effective treatment, antiretroviral drugs, that slow HIV's method of infection which is to copy its RNA onto cell DNA.

1. Well it is there just the sun makes it too bright to see. Stars are there are night as well but due to the numerous light on Earth we can't see a lot of them. Doesn't mean they haven't gone anywhere.

2. We have satellites in space. I'm pretty sure it would be spotted.

http://news.bbc.co.uk/2/hi/8272113.stm

They're getting there with a vaccine.

Okey dokey.

New questions.