Salvage them at the Scrap Yard and they end up in your Family Inventory. If you have a big enough lot (see semi-expalantion/cautionary tale next), take them out and pack them really tightly together. Leave just enough room for your Sims to get to them. Have your Sims detonate the innermost, and largest, item. Usually, if that item is large enough (maybe it's bulky, as I have had mixed results) then all the surrounding stuff will become collectible Scrap as well.
I stumbled on this method as I was detonating the really large pile at the Scrap Yard. Two cars were parked on the lot and literally everything blew up! IIRC, my yield was almost 3,000 scrap. My Best Friend eats well, now, and I haven't gone done much besides fiddle around with Detonating since.