Green plants will grow towards a light source. Walking under a ladder will cause bad luck. The event under study, or evidence of the occurrence of the event, can be observed and explained. The observations are limited to the basic human senses or to extensions of the senses by such things as electron microscopes, Geiger counters, etc. If the phenomenon cannot be reproduced through controlled conditions, natural evidence of the event's occurrence must be available for investigation.
Some plants eat meat.
Extraterrestrial beings have visited Earth. Scientists may not use supernatural explanations as to why or how naturally occurring events happen because reference to the supernatural is outside of the realm of science. Scientists cannot conduct controlled experiments in which they have designed the intervention of a supreme being into the test. Green plants convert sunlight into energy. Each prediction can be tested to determine if the prediction is true of false.