Put one cup of water in a small saucepan with the agave or maple syrup. Turn the heat to medium high (5 or 6) and let it heat up for 3 minutes or so. When all the maple syrup has been dissolved, immediately take the saucepan off the stove.
Add the cup of cold water and lemon juice and stir until everything is mixed very well.
Put the lemonade in the fridge (or freezer if you are impatient) for a while to chill, and serve with ice and lemon slices if desired.