Chondrules, small spherical inclusions inside a stony meteorite, are usually less than 1 cm (0.4 inch) in size, but they provide huge clues to the formation of our solar system. The silicate minerals olivine and pyroxene are the most common ingredients of chondrules, though their outer layers may consist of feldspars or other silicates. Scientists think that chondrules started out as aggregates, or clumps, of small, primitive dust particles, which were quickly heated by some cosmic event - perhaps some outburst from the young Sun, or simply the friction from small bodies ("planetesimals") crashing into each other. Some chondrules contain even smaller grains of silicates that may be some of the oldest materials in the solar system. Scientists measure the amounts of trace radioactive isotopes in chondrules and meteorites in order to estimate their ages.