As a gas, hydrogen is a very low density material and difficult to store. (scientificamerican.com)
It's hard to get an X-ray image of low - density material like tissue between bones because X-rays just pass right through like sunlight through a window. (sciencedaily.com)
BUMPER FILLERS, PRIMED, SET OF 2 — Made of high - density material for maximum durability and reliability; Most affordable solution for your replacement needs; Backed by 1 - year, unlimited - mileage warranty. (carparts.com)