A hypothesis is a statement that might be true, but which needs to be tested.

For example, Al forwards the hypothesis that "pink snails move more slowly than brown snails."

Al will be able to test his hypothesis by calculating the speed of hundreds of pink and brown snails, and then analysing the data he has collected to decide which colour of snail is the fastest.

Sometimes scientists will forward hypotheses that cannot be tested, but which seem to be a good explanation of something. It is worth noting that such hypotheses may be wrong. For example, an archaeologist might put forward the hypothesis that an ancient people believed that a certain rock painting was endowed with magical powers. Without a time machine, this hypothesis cannot be tested, but it could be a plausible explanation for the size and position of the rock painting.