Recently my students have been doing lesson with the /c/ and /k/.  They have discovered that when using /k/ it must follow the letters "i", "e", "y" & when using /c/ it must follow "a", "o", "u", and consonants.  We soon come upon words such as "skate", and "skull", and "skunk". I was not sure how to explain it to them.  Upon doing a bit of research I found a few things but I am not sure if I am correct.  If anyone has any suggestions on how I can explain this to my students it would be most helpful. 

I took a look at the etymology and found that that these are words that are borrowed and are widely used in the English language.  This especially refers to words that begin with "sk". For example, skate came from the Dutch language, skull came from Scandinavia, and skunk is a Native American word.  

I was reminded  of graphemic signals of language of origin like

<ch> for /k/ and and internal <y> are signs of Greek
<igh> and <ugh> are signs of Old English as is the <kn> for /n/.
<ch> for /ʃ/ is a sign of French...
I wonder  if the <sk> letter sequence have a similar origin from northern Europe?
What do you think?