Step 1: Materials
3. Ice cube tray. Usually comes in same box as freezer.
Step 2: Procedure
I found that it works best if you only fill the ice cube wells half way.
This website has an explanation of why they occur. Contrary, to what the website says, I found the best spikes formed when my freezer was set to normal (about -10 degrees C). My freezer does have a fan, but the temperature control is not very accurate.
This could make an interesting, safe, science fair experiment for kids. They can change the variables to see what has an effect.