Lentil Soup

I once had the best tasting lentil soup I had ever had, so of course I inquired how it was made. Her family was from Greece and she said there's 1 secret ingredient for lentil soup. It adds acid and sweetness, can you guess what it is? I said vinegar? No. Uh Salt? Duh....No Give up? Ketchup....add just before you finish cooking, just a few squirts and presto - acid from vinegar, zing from tomatoes and sweetness from the sugars. Works every time!