Watermelon and rum cocktail

We had a watermelon which needed eating and some rum that needed drinking... what else were we going to do?

1/2 watermelon
3 lemons
1 measure Cointreau
1 measure grenadine (or just a bit of sugar if you don't have)
4 measures rum (or more...)
Some mint leaves
Dark brown sugar (or other if you don't have)

Juice the watermelon (you can blend and put through a sieve or use a juicer - I pressed through a seive as the juicer is lost in one of the many boxes but that is an effot)
Juice the lemons
Mix everything together except the sugar and keep back a sprig of mint if you want it to look nice.
Make sure you crush the mint some to let the flavour out.

Serve over crushed ice with brown sugar and a sprig of mint on top. Or do as we did and put the mixture in an icecream maker until it turns in to slushy then serve with brown sugar and sprig of mint on top