Explain how a grain of sand that wash up on a beach could end up some day as lava pouring from a volcano?

The grain would have to be subducted into a plate boundary so that means maybe it got washed away from the storm of waves. Then it would end up in a oceanic trench. The sand grain and all of the other particles get buried and pulled deeper and deeper. They become metamorphosed and then, as the temperatures continue to rise, they finally melt. That melt is called magma and is lighter than the surrounding rocks because it is liquid. That magma can make its way to the surface to create a volcanic eruption!