Slice the lemons into thin slices, remove all the seeds. Place the slices into a large bowl or pitcher. Cover with the sugar. Let it sit for 10 minutes. After the 10 minutes are over, using a wooden spoon, mash the lemon slices and sugar together until it's mostly pulp. Stir in the seltzer water. OPTIONAL- Strain into another pitcher to get rid of pulp. Add ice.