Wash the watermelon and peaches thoroughly. Remove the rind from the watermelon and the pits from the peaches. Chop both fruits into smaller pieces.
Blend them until you achieve a smooth purée.Add the lemon juice to it.
Pour the fruit purée through a fine-mesh sieve or cheesecloth into a clean bowl. Press firmly to extract all the clear juice, leaving behind any pulp. You should have clear fruit juice.