Mixed references in a grid

The dollar signs are independent. $B$2 locks both parts, B$2 locks only the row, and $B2 locks only the column. Those middle two are mixed references, and they are what let one formula fill a two-dimensional table.

The trick is to ask which way each reference needs to move. Filling right and down across this grid, the headings along the top should stay on row 2, and the headings down the side should stay in column A.