Mouth ulcers are painful and a nuisance, since they can last for up to 9 or 10 days. They are caused by a viral infection, by biting your tongue or by dentures that don't fit properly. If they recur or persist for 2 or 3 weeks, see your doctor.
Treat mouth ulcers with a mouthwash, or suck glycerine pastilles. Or rub glycerine onto the ulcers with a clean finger, to soothe the soreness.
Take one or two soluble aspirin (paracetamol for children younger than 12) to relieve the pain.
If you think the soreness may be caused by a jagged tooth or ill-fitting dentures, see your dentist.