This question already has an answer here:
Newton's laws states that for every action there is an equal and opposite reaction. That is why we do not sink in ground because we apply a force of mg on the ground and the ground responds with a reaction force whose magnitude is equal to mg but is opposite in direction. This makes us able to stand on ground but when we stand on sand then we sink in it beacuse instead of applying a reaction force equal to mg, the sand moves away. So my question is that if the sand does not apply a reaction force then does it violates Newton's third law.