how to modify a field only once?

[robo_obi]

I'm doing software design and developmetn at school and our first task is to make O's and X's using filemaker pro......I have the most functional game in the class (i was the only person who had used the program before) but I have one problem...you can modify the fields more than once eg. you can click on it to place a O in, but then it is possible to click on it again and place an x. My teacher has told me it is possible to make it so a field can only be modified once.....could anyone tell me how this is to be done?

thanx

[cjaeger]

see record based security and set records to read only if field X OR field O is not empty ...