"I win," Inez announces.
"Don't count your chickens," you tell her.
She shakes her head at you, grinning. "Just watch me." She takes her pen back and adds draws an X in Square i:
"I now have two in a row, so you have no choice but to block me by putting an O in Square a." She doesn't wait for you, entering the letter herself:
"… which gives you two in a row, forcing me to take Square c:
She leans back in her seat again, satisfied. "And I've won."
You frown down at the napkin. "No – I just put an O in Square f and I block you."
"And then I put an X in Square g and win."
Now you see it. "And if I block you at Square g, you just put an X in Square f –"
"– and win that way," Inez finishes for you. "In chess, they call this a 'fork' – where you are forced to block two attacks at the same time, which, of course, you can't. And I want to point out that you don't have to be an evil genius to set up this little trick. Once I placed my second X, you had to take Square a, which forced me to take Square c, which set up the fork. As soon as I placed that second X, there was no possible way for you to win."
Frowning, you snatch back the pen and pull a fresh napkin out of the holder. But a few minutes of trial and error shows you she's right.
"Okay. What's your point?"
She pulls out a third napkin and draws a fresh grid. "This time, put your O in Square a instead:"