A cause is an event during which the transfer of energy from one configuration to another is changed. A hammer-blow event is the cause of the nail penetrating the wood because the energy of the hammer was transferred directly via the nail to the wood. A drought may cause a famine if the level of food energy available to the local population is so diminished that individuals are unable to obtain adequate nourishment. Such events are often simplified and interpreted as a single transfer-change event but in fact they are always participants in a sequence of events. Each cause can be viewed as a trigger event within a cascade of energy-change transfer events, or as an umbrella event encapsulating many event-stages. An explosive terrorist event precipitates a cascade of energy events, which eventually includes altering the neuron pathways of the brains of the survivors to provoke anger and initiate planning for revenge. It is also usually the case that a set of one or more events or circumstances are influential on the outcome of one or more other events.
Determining which are necessary conditions and which might be contributory to causal events is frequently an investigation of some difficulty. The cause of all the fireworks might be supposed to be the gunpowder, or the fuse igniting the explosives, or the message which came down the optical fibre, or the resentment of the mouse which got its nose whacked, or the attraction of the diode bait, or the energy stored in the coiled spring of the trap, or.... In fact all of the factors mentioned need to be included in a thorough description of the event.
A consistent event sequence does not necessarily indicate a causal relationship, even though a so-called "effect" always follows the so-called "cause". Many phenomena are cyclic and to suppose that one phase "causes" the next phase is quite unhelpful. Thus night does not cause day, nor low-tide cause high-tide, nor recession cause growth, even though the former always precedes the latter...or vice-versa depending upon your starting point. One o'clock certainly doesn't cause 2 o'clock unless you are touring Wonderland. The setting of the sun however, can be thought of as causing night, because the arrival of local light energy has changed to a much smaller value, and the consequent visible environment is what we call night.
Correlation is not a defining characteristic of cause either. The statistical population increase/decrease of a biological species A that correlates to the statistical changes of a species B does not indicate that either species "causes" the changes in the other. Both could be responding independently to something else...like climate change for example. The statistical changes in carbon dioxide concentrations that are correlated to the statistical changes in average global temperatures does not indicate that either causes the other. Both are more probably responding to changing solar and cosmic energy inputs.
Survival depends upon acquiring an extensive data base of highly probable causal relationships and yet in the chaotic environment of our existence, the smallest of energy transfer events can occasionally cascade catastrophically to precipitate a totally unexpected outcome. Nature will always surprise us thus and forever keep analytic predictability an impossible aspiration.