Ice will sometimes form spikes when it freezes. I couldn't believe it, so I had to try it for myself.
Step 1: Materials
1. Distilled water. You can buy this at the drug store or grocery store
3. Ice cube tray. Usually comes in same box as freezer.
Step 2: Procedure
It turns out that if you freeze pure water, it will sometimes forms spikes as shown in the picture. Any impurities, such as in tap water, seem to inhibit growth.
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.