Agood experimental design should always

A good experimental design should always BE REPEATABLE.

When scientists report the results of their investigations, other scientists always critically examined their works in order to confirm if the results obtained are valid or not. To accomplish this, other scientists have to carry out the experiment just the same way that the original investigator carried it out. This implies that, it is very important for scientists to design their experiments in such a way that it will be very easy for other scientists to copy and reproduce. Scientific experimental set ups must be repeatable.

gas

explanation:

smoke is the matter of a gas