From: Julie, a parent

My 8-year-old son is in the 3rd grade and they are studying geometry.  Last year in 2nd grade when the teacher asked in class how many sides a pyramid has my son answered 4 (4 sides and a base that is) thinking of the pyramids of Egypt and the teacher told him no that it had 3 sides and a base.  My son begged me not to argue with the teacher so I dropped it.  This year the question about how many sides a pyramid has is play again but the accepted 3rd grade answer is 4 sides and a base.  My son maintains (and I agree) that a pyramid can have as few as 3 or many sides, for example with a stop sign as a base one can make an 8 sided pyramid.  He is very good for his age at visualizing these things and I don’t want him to learn something that isn’t the complete picture.

Hi Julie.

You have a bright son - he's right! A "pyramid" is simply a polygon base (any number of sides) with triangular sides meeting at a common point (the "vertex").

Mostly, when we say "pyramid", we mean what are called "regular pyramids". Regular pyramids are ones where the base is a regular polygon (all sides and angles the same) of any number of sides. The simplest regular pyramid then is a 4-sided pyramid (base + 3 sides). Its proper name is a "tetrahedron". The tetrahedron has the extra interesting property of having all four triangular sides congruent.

An Egyptian pyramid has a square base and four triangular sides. Its proper name is a "square pyramid".

Good for you!
Stephen La Rocque.