G.L. c. 140, § 155

If any dog shall do any damage to either the body or property of any person, the owner or keeper, or if the owner or keeper be a minor, the parent or guardian of such minor, shall be liable for such damage, unless such damage shall have been occasioned to the body or property of a person who, at the time such damage was sustained, was committing a trespass or other tort, or was teasing, tormenting or abusing such dog. If a minor, on whose behalf an action under this section is brought, is under seven years of age at the time the damage was done, it shall be presumed that such minor was not committing a trespass or other tort, or teasing, tormenting or abusing such dog, and the burden of proof thereof shall be upon the defendant in such action.