Why do fresh cut lettuce go limp but when you buy it from the supermarket it is crisp?

Don't cut lettuce with a knife; tear it instead. Before you refrigerate it, put it into a plastic bag and twist to seal. It will stay crisp for quite a while. The same technique works for celery and other vegetables. There are special lettuce containers available but they're a waste of money; a simple plastic bag does the job just fine.