Fruit and vegetables can be costly things, and you're often buying by weight when you won't eat the whole thing anyway. In the case of lettuces, onions and celery, it's feasible to regrow a lettuce from the bits you may also throw away.
Back To My Roots covers the process of regrowing a lettuce from the stump, noting that all you really need is a bowl and some water for it to grow in, with daily water changes. It would seem wise if you wanted to adopt this larger scale to transfer to soil at some point purely to enable the lettuces you grow to gain a few nutrients along the way. Re-grow Romaine Lettuce Hearts [Back To My Roots]