"Lemon" is almost always used to describe cars. Also, a piece of junk is just something that isn't any good and doesn't work. Maybe it worked well when it was new, but now it's old and broken. However, "lemon" always means something you bought NEW but immediately had problems or stopped working. Calling a product a lemon can also imply that the seller deceived you.

To me, a "lemon" is only used in the sense of a low-quality or badly-damaged car. From Wiktionary: "He didn’t realise until he’d paid for it that the car was a lemon." and from an email I got a few days ago: "he sells cars at very reasonable prices, he wouldn't sell you a lemon!" It carries the same meaning of "piece of junk", albeit that that phrase is quite American in its usage. Commonwealth English speakers would be more inclined to say "it's rubbish".

These two are American expressions and nearly have the same meaning. Look at below: - What do you do if you buy a new car or a used car and it is a lemon? " A LEMON is a nice way of saying the car is A PIECE OF JUNK. The car is continually breaking down and is not what the dealership promised it to be". So I think they may be the same; even you may use these two to explain about the bad quality of a product. :)

I think " a lemon" means the product is not the first grade. Maybe there is only a little bit of defects, this is also called this product is a lemon. And also if the quality is bad, it can be just like the junk. "junk" usually stands for the product can not be worst any more.

i think , no you cant change it to lemon . thats just what i think ok :)