If you drop a paperclip into a cup of water, it will sink to the bottom, because paperclips are more dense than water. But, if you're careful, you can take advantage of water's unique properties and make the paperclip "float" on the surface.
Fill a cup with water - the fuller the cup, the easier to do this.
Use a fork to gently place a paperclip on the water's surface. You want to place the whole paperclip on the water at one time. You don't want one part of the paperclip poking through the water.
If you get frustrated watching someone else get their paperclips to stay on the surface while you can't, place a drop of soap in thier water while they're not looking...
Water molecules like ot stick to one another, creating a membrane-like surface across the top of the water. If you place the paperclips just so, that strong surface tension will support the paperclips (they aren't actually floating - they are still more dense than water). Soap breaks water's surface tension, so you won't get soapy water to support anything.