Why do straws float when first placed in a glass of carbonated beverage?

Answer 1

In a carbonated beverage, the gas wants to separate itself from the liquid as fast as possible. So they rise to the surface. This creates an upward force, caused by both the tendency of the bubbles to escape and by the fact that gas is lighter than water (buoyancy). If a straw is placed in this bubbly fizzy goodness, the straw will experience the upward force caused by the millions of bubbles escaping. Therefore, it appears to "float".

Answer 2

When you open a container of a carbonated beverage, the pressure in the container is reduced. That reduces the solubility of carbon dioxide in the water, so it is released as a gas. The straw is a disturbance to the liquid, which acts as a catalyst to stimulate this release of gas. The bubbles adhere to the straw, and of course are lighter than the water they displace. Soon you have enough bouyancy to lift the straw.