Regrowing Lettuce in Water
Rainer Zenz / CC BY-SA (Cropped the image)
Once you buy Lettuce for your sallad or any kind of use, buy the Lettuce which has the entire plant not only just leaves.
Cut the upper portion as displayed in the below image and use it.
Rainer Zenz / CC BY-SA (Horizontal line on image)
While putting the bottom portion in a cup having water. Your half of the bottom portion of Lettuce should be submerged in water. In a couple of days you can see the growth of the leaves from the bottom portion. Keep changing water every alternate day. You can harvest the leaves 2-3 times from your new grown Lettuce.