I'll tell you later today - just post your guesses as comments - you'll be so annoyed that you couldn't get it - it is simple and something you probably use every day.
Well the chemical formula of water is H2O Change the meaning (but not the sound) of the 2 and you get H-O which when written out in full if H I J K L M N O
:) Don't worry if you didn't get it - it doesn't mean you are thick.
Do you want me to do anoher one??? Peace (and frustration) peter