When we think of a container full of water we usually picture the water level being at the same height as the rim of the container. When you place another object in the container, it will displace its volume in water, or part of its volume if it floats, thus causing the water level to rise, and potentially spill over the edge. However, due to a property called surface tension a container can actually hold a significant amount more than the volume at which the water is level with the rim without spilling.
Liquid molecules like to stick together, this is called cohesion, and at the point the liquid touches something else, in this case the air, we call this surface tension. We can see if we very slowly overfill a container, a bubble like surface forms on the water, that's surface tension keeping it together rather than spilling out over the top. This is the phenomenon that causes water to tend to form round droplets rather than just spreading out in thin sheets, and can work in the other direction to keep small objects afloat despite being more dense than water.
I've included a picture of an American dime (about 18mm in diameter). As you can see, it has no significant depth to hold a liquid. Despite this, I was able to easily get it to hold over 20 drops of water forming a 3mm disc on top of it, all thanks to surface tension.
(published on 03/01/12)