This is an example of a TicTacToe starting grid. The start of a TicTacToe puzzle is normally
quite easy, there are two things to look for,
Look for either two 'X's or 'O's in a row (vertically or horizontally). Since it's not allowed
to have three (or more) 'X's or 'O's in a row, the cells either side must be the opposite sign.
In this grid we have two 'X's in a row, the highlighted cells above and below must be a 'O'.
Look for two 'X's or two 'O's separated by one empty cell. This empty cell in the middle must
be the opposite symbol. If the empty cell wasn't the opposite symbol, we would have three
symbols in a row. In this grid we have two 'O's separated by just one cell, this highlighted
empty cell must be a 'X'.
We can apply these rules repeatedly in the initial phase of a TicTacToe puzzle.