Hi Alex. The pressure that our bodies put on the blades causes a very thin layer of the ice to actually melt under our feet. The ice melting means there is a little bit of water under the metal blades. This thin layer of water is slippery enough for our skates to glide easily, which lets use skate fast without a whole lot of effort.

Metal skates are good for ice skating because both metal and ice are slippery. When metal is polished - made as smooth as possible - it can really slip, even on carpet. Ice is much more slippery than carpet, so when two slippery things - smooth metal and smooth ice - come together, ice skating is made possible.

How slippery the metal is depends partially on how heavy the metal skates are. Since metal blocks are heavier than metal blades, metal blocks push down on the ice more, and this push makes the metal blocks less slippery than the metal blades, so we use blades that are as sharp as possible for ice skating.

Ice is slippery because the ice skate's blade pushes on the ice and melts it a bit, to make water, which is easy to skate through.

Here's a link for kids about ice skates: ice skates

How to skate fast

Why is ice so slippery?

Liquid water is denser than frozen water, which is unusual; most materials are less dense when molten than when frozen, but water is the exception. This means that pressing on solid water (ice) will cause the ice to melt. Water on ice is slippery, but if the place where the pressure is applied is very small, such as the underside of an ice skate, then the skate will only slip in a controlled way.

This is how skating works: the blade of the skate melts the ice underneath it by applying pressure.

There is still some debate over this, but the current view is that ice is slippery because of a layer of water molecules present at the surface. These water molecule roll between the ice and the object (here, an ice skate) pressing against the ice, like tiny ball bearings. [For more advanced readers, a recent journal article is here .]

The force required to "roll" these water molecules is much lower than that to slide of the skate against a dry surface. This liquid layer decreases in thickness with temperature, becoming practically nonexistent at ~-100°C (212°F). This means that ice below that temperature is not slippery. In addition to this "liquid-like" layer, there is probably an additional contribution from friction between the skate and the ice. Although the friction between the skate and the ice is low, it is not nil. As a result, sliding against and deforming the ice produces some heat which may melt a small amount of ice and contribute to the lubricating layer.

One hypothesis for the low friction which has been quite conclusively disproved is that the top layer of the ice melts because of the pressure of the skate blade. The explanation for the hypothesis comes from an examination of the phase diagram of water. [A phase diagram is basically a plot showing combinations of environmental conditions where different phases of a substance, e.g. liquid water, ice, and water vapor, are able to exist.]

Water is one of a small number of substances which has a negative slope to the boundary between the solid and liquid phases. This means that ice can be made to melt at a temperature below 0°C (32°F) by increasing the force pushing down on it. This force might come, for example, from an ice skater. While this will work in theory, the temperatures of ice rinks is low enough that the pressure increase required to melt the ice is far greater than that produced by any ice skater. Thus, this cannot be the explanation.

[Here is an article which discusses the competing theories. It is beyond a first grader, but is not so scientific as the journal article.]