A private nuisance is an act or condition on the defendant's land that substantially and unreasonably interferes with the plaintiff's use and enjoyment of plaintiff's land. The interference may be an intangible invasion such as smells, light, sounds, vibrations, dust, and pollution of air and water rather than a physical invasion, which is subject to strict liability in an action in trespass. For instance, a person walking his dogs on his neighbor's land trespasses and is liable for at least nominal damages. If that same person allows his many dogs to bark all night, the barking dogs may be a nuisance if a court determines the barking substantially and unreasonably interferes with his neighbors' use and enjoyment of their property. Although both trespass and nuisance are actions to protect possession, trespass is more easily proven than nuisance: Only the invasion need be shown in a trespass action. Trespass is not concerned with the substantiality and unreasonableness of the invasion.