Why do stars twinkle at night?Explain.

The air near the earth being always in motion due to convection currents, its density does not remain constant at all the points, it goes on changing, even at points in the same level and therefore, the refractive index at different points goes on changing. So, the rays from a star  travelling in a given directiuon are somtimes concentrated at one point and sometimes at another which results in the twinkling of stars.