Complex sentence: Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings; unless they are cylindrical, they must also contain pentagonal (or sometimes heptagonal) rings.
Split into simpler sentences: Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings. Unless they are cylindrical, they must also contain pentagonal (or sometimes heptagonal) rings.