That's exactly the problem I ran into when Fishing Cactus asked us to review Equilibrio (click opens iTunes), their new ball puzzle game for iPhone and iPod touch. The app, which is also available for Nintendo Wii, is an addictive game in which you roll balls made of various materials (rock, paper, steel, rubber, and more) around a maze using the iPhone's accelerometers to control where the balls go. You can gain extra points by hitting "coins" and zipping to the end of the maze quickly. There is a conquest mode, where players need to complete a series of levels, as well as a challenge mode. The app is currently available in the App Store for half-price at US$1.99, and there's also a free version -- Equilibrio Lite -- available if you want to try before you buy.

Fishing Cactus has supplied us with 5 promo codes to give away to lucky iPhone / iPod touch owners in the US and Canada. To enter the giveaway, leave us a comment with your idea for a totally different material to make an Equilibrio ball out of.

Here are the rules and a link to the obligatory legal statement:

• Open to legal US residents of the 50 United States and the District of Columbia who are 18 and older. (Sorry, we know our international readers want to participate, but promo codes don't transfer outside the US.)
• To enter leave a comment with an idea for what material you think a Equilibrio ball should be made of.
  
• The comment must be left before Thursday, June 12, 05:00 PM Eastern Daylight Time.
• You may enter only once.
• Five winners will be selected in a random drawing.
• Prize: One license each for Equilibrio (US$1.99 value)
  
• Click Here for complete Official Rules.

Good luck! And check out the gallery below for some screenshots of this addictive iPhone game.