Water has air dissolved in it. This air forms the bubbles you see when you start to heat water (as opposed to the bubbles formed by water vapor when water boils).* You can remove most of the air by boiling the water. So I boiled a pan.
After the pan had returned to room temperature (I helped it along by letting the full pan of hot water sit in a cold water sink), I filled cups with 1/2 cup each of normal tap water and boiled water. Here's what happened:
In my tap water cup, bubbles quickly developed on the hearts. In my boiled water cup, so few bubbles formed initially that I could still count them. Two of my tap water hearts were floating within an hour.
Floating hearts covered with bubbles.
The hearts in boiled water, on the other hand, took much longer. It was an hour and a half before one of the hearts was finally covered with enough tiny bubbles to start floating.
Why did the heart in boiled water float at all? Because even though I boiled the water, the water was continually exposed to air. I couldn't remove all the air unless I tried the process with vacuum equipment--which I don't have in my kitchen!
*For more information, visit The Engineering Toolbox.