The invisible M...

Quizzing him on his spelling words while driving home...

Me: "How do you spell adjust?"
E: "A-D-J-U-S-T."
Me: "Awesome. Crucial."
E: "C-R-U-C-I-A-L."
Me: "Unpredictable."
E: "U-N-P-R-E-D-I-C-T-A-B-L-E."
Me: "Great! Problem."
E: "P-R-O-M-B-L-E-M."
Me: "Not quite. There's only one M."
E: "P-R-O-M-B-L-E...but it doesn't make sense without the M at the end."
Me: "Yep. That's the one that should stay."
E: "Ooooh! The M in the middle is invisible?"
Me: "Nope. No M in the middle. No M sound in the middle, either. Pro-blem."
E: "Prom-blem. See? There's an M sound."
Me: "Pro-blem. Not Prom-blem."
E: "What? Since when?"
Me: "Since forever. It's always been Pro-blem."
E: "Sometimes I just don't understand the English language. It seems like things are always changing."