What to do with an extra twin mattress
It is always a good idea to keep an extra mattress in your house no matter what size it is, there may be a point where you're sleeping on your mattress and a spring pops out and stabs you, making it uncomfortable and causing harm to you, so having an extra mattress will give you a temporary fix until you purchase a mattress that is suited to you.
Place in a guest room
If you have an extra twin mattress and a spare room that guests can use, why not store your extra twin mattress there, the room doesn’t get used much as others and if you do end up having guests over there's an extra mattress in case you need the space.
You can save the extra mattress just in case you need to replace an older twin mattress it is always a good idea to save an extra twin mattress, you rather be safe than sorry. Twin mattresses are a great temporary solution therefore t is a good idea to keep the extra twin mattress you may have laying around, but make sure it is encased in plastic as you wouldn’t want dust to get on it. You would want to keep it clean and prolong the lifespan of the mattress.
If you have an extra twin mattress and you do not know what to do with it, you have no use for it and you have no storage place for it, you may think it is a good idea to sell it, but before deciding to sell it make sure it is in great condition, this way you can get some money from it and you won't have to worry about clutter.