"You see the simplicity of it, and the brilliance. For Moody never does drink except from his hip flask, he's well known for it. The imposter needed, of course, to keep the real Moody close by, so that he could continue making the potion. You see his hair ..."
-- Dumbledore on The Fake Moody using Polyjuice Potion (GF35)
Mad-Eye Moody was well-known to drink only from a hip flask he always carried because he feared being poisoned by Dark Wizards even while among friends (GF12).
- It's unknown what drink Mad-Eye preferred to carry in the flask, only that Harry noticed he would never touch another drink, either pumpkin juice at Hogwarts (GF12), or mulled mead from Madam Rosmerta at The Three Broomsticks pub (GF19).
- The Fake Moody abducted the real Mad-Eye and kept him locked in the seven-lock-trunk while he made Polyjuice Potion from his hair. So the Fake Moody was swigging Polyjuice from the hip flask the whole time to protect his disguise, which even fooled Dumbledore (GF35).