Therefore, if you were to fill a 1-litre soda bottle full of helium, the bottle would weigh about 1 gram less than the same bottle filled with air. That doesn't sound like much -- the bottle itself weighs more than a gram, so it won't float. However, in large volumes, the 1-gram-per-litre difference between air and helium can really add up. This explains why blimps and balloons are generally quite large -- they have to displace a lot of air to float.

If you put helium in a balloon and let go of the balloon, the balloon rises until it pops. When it pops, the helium that escapes has no reason to stop -- it just keeps going and leaks out into space. Therefore, in the atmosphere there is very little helium at any given time. The helium that is there comes from alpha particles emitted by radioactive decay. In places that have a lot of uranium ore, natural gas tends to contain high concentrations of helium (up to 7 percent). This makes sense, since the decay of uranium emits lots of alpha particles and a natural gas pocket tends to be a sealed container underground. Helium is cryogenically distilled out of natural gas to produce the helium we put in balloons.

What causes helium balloons to lose their lift after a day or two?
In brief, because the helium leaks out, they shrink, and become heavier than the volume of air they displace. This causes them to lose buoyancy and "sink" in the air. The weight balance that keeps a balloon afloat does not leave a lot of room for leakage, so once a little leaks out the balloon falls.

Sometimes you can catch a balloon right around the time it is neutrally buoyant, and applying heat (your hand, for instance) or cold (rub with ice cube) will change its volume just enough to make it rise or sink in the air.