Place the marshmallows and water in a microwave proof bowl. Microwave on high for 2 to 2 1/2 minutes until melted. To ensure even cooking, stop the microwave every 30 to 45 seconds and stir them.

Stir 1 to 2 cups of the powdered sugar into the melted marshmallows. Once that is incorporated, continue to stir in the powdered sugar 1 cup at a time.

Once the mixture gets too difficult to stir, grease your work surface and your hands with vegetable shortening and dump the marshmallow mixture out onto the work surface.

Knead more of the powdered sugar into the marshmallow mixture, you may not need to use all of the sugar. Work in enough sugar until the mixture feels like children's modeling dough. Keep your hands well greased with the vegetable shortening to prevent the fondant from sticking to them.

You will know it is done when it is smooth and pliable and stretches without breaking. If you add too much powdered sugar it will become stiff and crack, if that happens just add a teaspoon at a time of water to it and knead it in. If it is too sticky, add more powdered sugar.

Shape the fondant into a ball and coat it with vegetable shortening then wrap it tightly in plastic wrap and place it in a sealable bag. Store the fondant at room temperature and always keep it tightly sealed.

You can use the fondant right after mixing it but it will be easier to work with if you let it sit overnight. After it sits, it will firm up and you will need to knead it to make it pliable and soft again.