A naked single is the simplest deduction in Sudoku: a cell whose row, column and 3×3 box already contain eight of the nine digits. The missing digit is the only legal value.
Look for cells where most of the surrounding cells are filled. The fewer empty peers a cell has, the easier it is to find a naked single there.
There are three empty cells in this practice board. Each one is a naked single — find them and place the right digit. Use 1–9 on your keyboard, or tap the number pad below.