cake or pie?

What's the difference between a cake and a pie? The other day I was teaching some vocabulary related to food and my students asked me the difference between a cake and a pie. I didn't know what to say.

Thanks a lot,

Maite.

Cake on the left; pie on the right.
Cake is made from a mixture of flour, sugar, butter, eggs, and a raising agent. 
It has a sweet topping called frosting.

Pie has a filling of fruit or custard in a pastry shell.

Note this exception:  What is traditionally called "Boston Cream Pie" is actually a cake.

CJ