You answered this problem very well. Toby and Jodi from King's Park Primary School sent us the following:
First of all we decided which letters were symmetrical (D, R, U and N) and wrote these down. We then used these letters to make a word which we thought would be used in a semaphore message - RUN! We also made up the word - nun.
Some of you didn't pick up on the letter D being symmetrical. Remember, a person standing behind would still see just one flag up in the air. Rachael from Ardingly College, and Chris and Joseph from Moorfield Junior School, also found the word "urn" which, as Chris and Joseph pointed out, is an ancient Greek pot.
So that's three words in total. If you find any others, do email us.