Place the tahini and garlic in the bowl of a food processor. With the processor running, add the water in a slow stream and process until the tahini is light and fluffy, about 1 minute. Add the lemon juice and the salt and process 30 more seconds. Stop the processor, taste the tahini, and add additional salt and lemon juice to taste. Stored in the fridge, tahini keeps for up to 1 week. When you take it out of the fridge, thin it with water or lemon juice and season with salt to brighten it up.