When I was 7 or so, my mother and I took a trip to the UK. We bought a bottle of some orange flavored drink to take back to our hotel because I enjoyed the stuff.
For some reason, the store-bought stuff was pretty hard to get down and seemed a bit sweeter and more viscous than would be desirable. It took me about a week to realize that I was drinking undiluted orange-drink concentrate.
Upon reflection, it was within a year or so of that trip that I was diagnosed with ADD and began being medicated for concentration problems.