Mom, I'm thirsty!

Fine when you hear this in the daytime - just give them a drink. But when you hear it after you've tucked them into bed.....
You have a dilemma. Are they genuinely thirsty or just trying to keep you in their room a bit longer?
Do you get them a drink and risk them trapping you in their room for another 15 minutes while they sip slowly and talk at you? (or if they're like my daughter they gulp it back and keep doing 'pretend gulping' even after the water is long finished!)
Do you say "no darling, go to sleep" then spend the rest of the evening racked with guilt that they are lying in bed absolutely parched?
My little one was a great sleeper as a baby. I'd put her in her crib, she'd roll over and miraculously go to sleep! Then when she was in a toddler bed they shared a room. This was a nightmare as they kept each other awake for hours! We had a rough time for several months after moving house last summer when she went back to having her own room. She would cry for hours and come downstairs repeatedly. Now she's used to having her own room she just wants to climb in to bed at the end of the day, roll over and go to sleep - just like when she was a baby.
My oldest though is a different story. She has always used stalling tactics at bedtime. "lie with me"; "just one more book"; "send Daddy up to say goodnight"; "where's my cuddly toy/doll/blanket...".
Now she's started with the "I'm thirsty" line. I try to make sure she drinks enough in the day and let her have a little drink after brushing her teeth but she really has me with this line. You know yourself how horrible it is to feel thirsty!
So, she usually ends up getting her way. I return to her room in response to "Mom, I'm thirsty!" and end up talking to her, lying with her, finding another toy (to add to all the toys already at the bottom of her bed!), putting the fan on, sending Daddy up, starting her music again.......then collapsing in an exhausted heap and hour later than I meant to!