A very simplistic, but fairly adequate description of surface tension involves the surface molecules of a liquid, most importantly water. It is created when the molecules of the water, i.e. H2O, on the surface bond tightly together; much tighter than those below the surface. This allows the surface to behave as if it is a weak barrier, of sorts, that can have unusual properties.
(Image from: https://www.usgs.gov/special-topic/water-science-school/science/surface-tension-and-water?qt-science_center_objects=0#qt-science_center_objects)
One of the properties that this causes dictates the shape of the water's surface; it will take the form that creates the least amount of surface area as possible. This is why water will bead up, having a round surface, because this minimizes the amount of water that is exposed; a bead of water has less surface exposed than a sheet of water created with the same amount. In a weightless environment (where gravity is canceled out), water will form a sphere, or ball, because that is the shape that produces the least amount of surface area.
You can see this phenomena in this video of an astronaut on the International Space Station (ISS), who is in a weightless environment, playing with water. You can clearly see the spherical shape here. It even keeps the shape, mostly, when a tablet of alka seltzer is inserted in it and it starts to bubble (effervesce).
Now, for today's experiment: if we're careful, we can use water tension to float items that would normally sink in water. It has to be lightweight, but it does work. For example, everyone knows that a paperclip will sink if you put it in a glass of water, right? Well... usually. There are a couple of techniques that allow us to float a paperclip on the surface! Let's give it a try. Note: you may have to be patient; this takes a little bit of skill.
What you'll need:
A glass, with a wide mouth, of water
A piece of tissue big enough to hold the paperclip, but not bigger than the mouth of the glass
Placement method #1: Put the flat paperclip on the tissue, then gently lower the tissue onto the surface of the water. When the tissue is wet, gently force it down with the toothpick so that the paperclip is left floating on the surface. If it doesn't work the first time, you need to dry the paperclip thoroughly before trying again.
Placement method #2: Bend one paperclip into a right angle and use it as a holder. You will rest the other paperclip on it, then you can lower it to the surface of the water gently. Again, you need to dry the paperclip well if you have to try again.
This video shows how both of these methods are done:
For something really cool and fun, check out these astronauts playing with a big ball of water and a camera.