Secret Runes
Tiles that change in the game are known as secret runes. These secret runes change from blue tiles to tiles with evil faces. They can occur at any point, but are most common in the final level: Alexandria.
Significance
The secret runes are significant because they are easier to match to other runes on the game board. This makes it easier to get a series of four to five runes at one time. Not only will this increase your score, but it will also increase your chances of clearing the puzzle.
Activating the Secret Runes
To activate the secret runes on your puzzle board, aim to blow up at least one fireball and multiple lightening balls in succession. Match four runes in a row to get a fireball and match five runes in a row to get a lightening ball. Fireballs clear the entire row where the runes were cleared and lightening balls clear the entire row and column where the runes were cleared.
Drawbacks
Secret runes don't hold any particular value, nor do they have any special powers or effects. Their purpose is simply to make it easier for you to match the runes you currently have in the puzzle.