First time post so go easy on me :-)
I was reading with one of my curious 6 yr olds recently and they asked why 'busy' had a <u>.
I looked up the etymology and found this:

I would love to have something more concrete and memorable than "for some unclear reason".
I also looked at the Word Searcher, but it appears to be the only time <u> --> /I/.

My kids are also intrigued as to why <s> so often makes /z/. (is, his, as, was, these etc.)
We recently did an entire study into the sounds of <c> and they were so excited to find out about ci, cy and ce --> /s/.
Any advice about theZe examples would be greatly appreciated.
Many thanks,