Modeling ODE

Suppose that we drop a stone, which falls freely with no air resistance. Experiments show that, under that assumption of negligible air resistance, the acceleration ${y}^{\prime\prime}=\frac { {d}^{2}y }{ {d}^{2}t }$ of this motion is constant, i.e. equal to the so-called acceleration of gravity $g=9.8 \text{ m/sec}^2.$ State this as an ordinary differential equation for $y(t),$ the distance fallen as a function of time $t.$