"Wa" in Hiragana

Why is "wa" sometimes written as "ha" in Hiragana?

HA is usually a particle: the topic marker. When it acts as this particle, it's read as "wa". When it is a sound, as part of a word, it's read as "ha". Here's an example: はなは綺麗です - Hana wa kirei desu -> the flower is beautiful. The first ha is read as it is because it's part of the word hana(=flower), but the second one is read as wa because it's the topic marker for the word hana. Hope this clarifies things a bit, コッド