Arrow keys / WASD, or swipe on mobile, to steer. Eat the dot to grow — speed increases every time. Hit a wall or your own tail and it's over.
Neon Serpent is a free hard snake game online: steer a growing serpent around a 20x20 grid, eating dots while dodging the walls and your own ever-longer tail. The rules are the classic ones everyone knows — the speed ramp is what makes this one of the hardest snake games to play for very long.