Minesweeper

Right-click or long-press = flag

My record

Open every cell that has no mine to clear the board. Opening a mine ends the game.
A number tells you how many mines are hidden in the 8 cells around it.
The first cell you open is always safe and opens up a wide area.
Beginner is 9x9 with 10 mines, Intermediate is 16x16 with 40 mines, and Expert is 16x30 with 99 mines.

PC: click to open, right-click to flag, click a number (or middle-click) to open all its neighbors at once when the flag count matches.
Phone: tap to open, long-press to flag. Turn on flag mode to place flags with a tap. On Expert, swipe the board sideways to see all of it.