Question

why is it easy to burn a heap of dry grass compared to green grass​

Answer 1

because green grass contains moisture which lessens the efficiency of the combustion but heap of dry grass is free of moisture and allows easy combustion at a high rate

Answer 2

Heap of dry grass burnt easily as compared to green grass. This is because green grass contains some amounts of moisture and they are able to perform Photosynthesis so water must be present in the leaf of green grass but dry grass has no moisture and they are also not able to perform Photosynthesis. So that heap of dry grass burnt rapidly as compared to green grass.