What is Strength?
Some bricks are stronger than others. Stronger bricks can survive at higher altitudes due to stronger clay.
When you put a brick on top of a brick that matches it's color or put two bricks of the same color on the same place at the same time, you get a stronger brick. A brick's strength is shown in how many lines it has on it.
When you put a brick on top of a brick of a different color or put two bricks of different colors on the tower at the same time, you get a weaker brick. If a brick's strength falls below zero, it is destroyed.