沙

Left water radical (three abbreviated drops), side-form of 水, the indexing radical. It reflects the original sense: fine grains at the edge of water, where rivers deposit silt and sand. Same radical anchors 河 (river), 海 (sea), 湖 (lake). Sand was first noticed as a riverbank deposit, not a desert phenomenon.

Right side 少 supplies the sound (shǎo to shā, a tone shift) and a faint hint of tininess. Sand is mineral broken into the smallest grains, so water plus few or tiny is a near-pictorial spelling. Same phonetic in 抄 (copy), 炒 (fry), 秒 (second).