Buttresses!

Mr. Anderson's 7th graders are studying buttresses in gothic architecture like the flying buttresses pictured below: 

Or non-flying buttresses like these:

What's the purpose of a buttress?  Right!  To provide a lateral force to support a building...or a tree!  Did you know there are natural buttresses on many trees in Costa Rica?  Take a look...

Why do you think huge trees need crazy buttresses like these?  The answer is that in the tropics, the soil is super thin.   In biomes like the one we live in (who remembers the name of this biome?), the soil is pretty deep so tree roots can also extend way way down into the soil to keep a tree standing up in all kinds of windy weather.  Because  it's so warm and wet all the time in rainforest biomes, dead things rot really quickly and the nutrients get used up really quickly. The result is that not a lot of those nutrients end up hanging out in the soil for very long so the soil is very thin.  Buttresses support humongous trees like kapoks and figs and help them to remain standing even in very windy weather. Think of the hurricanes that pass through these areas from time to time!   Buttresses also trap dead leaves and keep them near the base of the tree so when they decompose, the nutrients are right there where they can be used by the tree.