1. This will be discussed by the example of the rule in Rylands v. Fletcher 1868
Who, for his own purposes, brings on his lands and collects and keeps there anything likely to do mischief if it excapes, must keep it in at his peril, and if he does not do so, is prima facie answerable for all the damage which is the natural consequence of its escape.
a) Things likely to cause mischief include: gas, electricity, fumes, rusty wire from a fence, explosions, a flag pole.
Crowhurst v. Amersham Burial Board 1878 - also gas and poisonous vegetation.
The rule does not apply to land itself: Davey v. Harrow Corporation 1957
b) There must be an escape: if some injury occurs on the premises, there is no escape:
Read v. Lyons & Co. Ltd. 1947 - female inspector hit by parts of an exploding shell while she was on the premises of an ammunition factory during an inspection.
c) However, the defendant's use of the land must be 'non-natural' (later regarded as ratio decidendi).
The bringing on land of ammunition was regarded a natural use of land during wartime as an obitur dictum in
Read v. Lyons & Co. Ltd. 1947

2. Defences
a) Plaintiff's consent to permit the defendant to bring the mischievous thing upon his property. Also were there is a mutual interest in the keeping of such a thing, such as a cistern used for water collection in house with several flats. See also: A. G. v. Cory Bros. 1921
b) Where the plaintiff's own fault has caused the escape.
c) Where the escape is due to the wrongful act of a stranger: Box v. Jubb 1879 - a third party caused the defendant's water reservoir to overflow onto the plaintiff's land by emptying his own reservoir which fed the defendant's.
d) If the escape is due to an 'Act of God', or vis majeur; but it's not enough to show that it was accidental. It must be exceptional.
e) Where the mischievous substance is collected by the defendant under statutory authority, such as a local authority carries gas or water in a pipe: Cambridge Water Co. Ltd. v. Eastern Counties Leather plc. 1994
These defences are not restricted to R.&F. - the first being an instance of assumption of risk, the second and third constituting a lack of causal connection, the fourth as an 'inevitable accident', though not exactly, and the fifth as a universal excuse, since an Act of Parliament can, in law, achieve any result (compare also public policy).