The historic notion of globular clusters as simple stellar populations is now proven to be too simplistic . In the last years a large number of observational studies have revealed the presence of large internal variations in their light-element content (e.g., C, O, N, Na, Mg, and Al). This extremely peculiar chemical pattern, which is not observed in open clusters and field stars, points to inter-cluster pollution processes and to the existence of multiple stellar generations. Although the first level of the game is clear, several fundamental issues are still open, related to e.g. the nature of the light-element distributions and to their relationship with the cluster structural parameters (such as mass, horizontal branch morphology). We present in this contribution lithium abundances for a large sample of giant stars in the globular clutter NGC 6218: thanks to its fragile nature (Li starts burning at only T~2.5 MK), lithium represents a very powerful diagnostic in this context and can provide strong observational constraints to the multiple population scenario in globular clusters. The comparison with models and the implications for cluster formation and internal pollution mechanisms is also discussed.