Scientific models and theories
A scientific theory is a simple or complex explanation put forward by scientists to explain various phenomena. A model is used to help scientists visualise things that they cannot actually see. It can be thought of as a thinking tool, to help form explanations. Models are useful simplifications to aid understanding.
The theory used is usually based on previous scientific knowledge and experiences, as well as careful observation and measurements. It can then be used to explain further phenomena and to make predictions of future behaviour. Scientific theories and models are powerful tools but they are only valid as long as they can explain all of the available data, ie from observations and measurements.