When you're thirty, you crave liquids. Thirsty people want a drink.
To be thirsty is to feel like you need to drink something. People tend to get more thirsty on very hot days or during exercise, when a lot of their body's fluid is being sweated out. Eating can also make you thirsty, and so can waking up in the morning. The word thirsty comes from the Proto-Indo-European root ters, or "dry."