Preheat the oven to 218�C.
In a saucepan over medium/high heat, combine the water, milk, butter, salt, and sugar. Heat until the butter is melted and the mixture comes to a boil.
Take the boiling mixture off the heat and pour all of the flour into the pot all at once. Stir vigorously until the mixture becomes smooth.
Flatten the mixture to the bottom of the pan and put it back on medium heat. Listen until it starts making a crackling sound. Pull the mixture back and check if there is a thin film on the bottom of the pan. If not, cook a little longer.
Remove the mixture from the heat and stir until all of the steam has evaporated off.
Off the heat, add one egg at a time and stir until it is completely absorbed into the batter before adding the next egg. When the batter is smooth and glossy looking, it is ready.
Move the choux pastry to a piping bag with an open round tip (or a ziplock bag with the corner snipped off). On a baking sheet lined with parchment paper, pipe the shells about 2.5 cm wide and 10 cm long. The shells will puff up significantly in the oven. Use a damp finger to smooth out the tails on the shells. They will not all fit on one baking sheet so wait to pipe the next batch until right before they go into the oven.
Put the shells into the oven and immediately increase the oven temperature to 232�C. The active heat will help the shells puff up. Bake at 232�C for 8 minutes. Drop the heat down to 177�C for 13-15 minutes, until the shells are golden brown and crisp on the outside. Let cool before filling.