So the architects of the Parthenon set out to make it the best temple ever. Most Greek temples had six columns across the front – the Parthenon has eight. Most temples had either a frieze (a continuous band of sculpture) or metopes (individual panels) – the Parthenon has both. There's a set of triglyphs and metopes on the main architrave, over the Doric columns. And there's a frieze on the inner architrave, over the Ionic columns. So while most Greek temples are either Doric or Ionic, the Parthenon is both.
Everyone wanted to make the building as beautiful as possible. Callicrates and Ictines wanted their Parthenon to seem to float, so they made the whole thing curve slightly upward to the middle, so it almost looks like it is trying to take off into the air.
And they knew that if you make the columns straight, an optical illusion makes them look thinner in the middle, so they made their columns a little thicker in the middle, so the columns would appear to be straight.
To find out more about the Parthenon, check out these books on Amazon or in your library:
A Greek Temple, by Fiona MacDonald, Mark Bergin (2002) (this is specifically about the Parthenon, not just any Greek temple)
Parthenon, by Lynn Curlee (2004). For kids. Lovely pictures.
The Athenian Acropolis: History, Mythology, and Archaeology from the Neolithic Era to the Present, by Jeffrey M. Hurwit (2000). This is not a children's book, but it is pretty interesting reading. Hurwit is a archaeologist and art historian who works on the Athenian Acropolis.