This produces about a litre and a half of dilutable fruit syrup. You can make it from any fruit that you can extract juice from: I've been successful with lemon, lime, orange, mandarin orange (I can imagine grapefruit but I don't like the taste so I've never tried), strawberries and pomegranates. A combination of citrus fruits works very well.
If your fruit isn't tart enough (strawberries!), you can adjust that with half a teaspoonful of citric acid or some lemon juice.
To prevent pulp in your drink, either strain the juice or use a slotted spoon to scoop off any particles of fruit that rise to the surface of the syrup while it's cooling.
The syrup will keep for weeks (at least) in the refrigerator. Dilute it 1:7 (or weaker) with (fizzy) water for a drink, or 1:5 to make ice lollies.
Make sure that the juice is in a large enough jug that won't burst when hot liquid is suddenly added to it. If you're using a glass jug, put it on a wooden board or a folded tea-towel.
Bring the sugar and the water to the boil and boil until all the sugar is dissolved. Yes, this is possible, even if the water doesn't look enough for the sugar. Add the citric acid if you're using it (lemon juice can go in with the rest of the juice).
Pour the hot sugar syrup into the jug with the juice and stir briskly. Let it cool to lukewarm and pour it into clean bottles. Refrigerate when it's cooled completely.