**1.2 Physical properties of chondrules**

The chondrule formation models, including the shock-wave heating model, are required not only to heat the chondrule precursors up to the melting point but also to reproduce other physical and chemical properties of chondrules recognized by observations and experiments. These properties that should be reproduced are summarized as observational constraints (Jones et al., 2000). The reference listed 14 constraints for chondrule formation. To date, there is no chondrule formation model that can account for all of these constraints.

Here, we review two physical properties of chondrules; size distribution and three-dimensional shape. The latter was not listed as the observational constraints in the literature (Jones et al., 2000), however, we would like to include it as an important constraint for chondrule formation. As discussed in this chapter, these two properties strongly relate to the hydrodynamics of molten chondrule precursors in the gas flow behind the shock front.
