Today my daughter (goes to kindergarten) asked me this question which made me post here? I felt that was a good question. Can anyone help me with an answer?
Explaining to a 5-year-old
English spelling is quirky. English spelling tends to be more influenced by how people spelled words over a thousand years ago than by how we pronounce the word today. Yes, that means that word pronunciation has changed drastically over time, and our language today is not the same as the language that was spoken back then.
One of those changes is that the /k/ sound, written using the letter C, came to be pronounced /s/ in some cases. That change probably happened back when people were still speaking a language called Latin (which nobody speaks anymore). The Latin word was circulus, and in the early days, both c's were probably pronounced as /k/. Then people found it too awkward to say "kairkooloos" and started pronouncing that word more like "sairkooloos", which eventually became our modern word circle.
The /k/ sound is one of the consonants that has a tendency to mutate over time, and that mutation even has a name: assibilation.
- It happened once in the ancient days of Proto-Indo-European, resulting in the centum-satem split. The phenomenon is named after the tendency for Indo-European languages towards the east to have a word for 100 starting with an /s/-like sound, and for the western languages to have a cognate word starting with a /k/-like sound. In Latin, the word centum was pronounced [kentum].
Originally, Latin had a letter C that always represented the /k/ sound. In the later days of Latin, the letter C would sometimes represent an /s/ sound when placed in front of some vowels. Confusingly, even though Latin was a "centum" language (meaning that it fell into the /k/ category in the k/s divide), its word for 100 eventually came to be pronounced with an /s/.
I believe that the word circle follows a similar history: originally pronounced in Latin with /k/ sounds, and later the first c came to be pronounced with an /s/.
- Assibilation happened in some words that evolved in Britain as well: witness kirk and church. I don't think that circle falls into this category, though.