Molecular crystals are solid materials formed by molecules held together by weak intermolecular forces, such as van der Waals forces or hydrogen bonds. These crystals typically have well-defined structures and can exhibit unique optical and electrical properties. Common examples include sugar and ice, where the arrangement of molecules influences their physical characteristics. The formation of molecular crystals occurs when molecules pack closely together in a regular pattern during the cooling or solidification process. This arrangement can lead to various crystal shapes and sizes, impacting their behavior in applications like pharmaceuticals and materials science.