What do a control group and experimental group have in common?

I need to know what a control group and experimental group have in common for some science homework. I already researched it and came up with nothing. Please help. I need this information by tonight.|||Whatever the focus of the experiment is, plus any others factors that might influence the outcome of the experiment. If you are testing a new cancer drug, the experimental group and the control group must both be people with the same type of cancer, and both be a representative distribution of the population, all races, genders, ages, etc. You want the only difference in the two groups to be what you are studying, i.e. the effects of the drug.|||It is essential that they have everything in common, EXCEPT the one variable you are testing. For example, say you want to know if putting an aspirin in the soil will make flowers more colorful. So you grow a group of plants, put an aspirin in the soil, and the plants produce very colorful flowers. What have you demonstrated? Nothing! Because there are dozens of reasons why plants might produce very colorful flowers - materials already in the soil, temperature, light, humidity, etc. What you should have done is to grow two identical sets of plants, same seeds, same pots, same soil, same water, same light, same humidity, etc. In one set you put an aspirin in the soil. In the other set - the control set - you don't. Now, if the set with the aspirin have more colorful flowers than the set without the aspirin, you have demonstrated something real because the control rules out all other possible explanations for the flower color.|||they have as much as possible in common, except the thing you are testing.