Suppose Yara had sprinkled half a spoon of fertilizer on each pot of the second group instead of none.

Is this group the control group? No, it is not: it got some of the change.

Two groups: fertilizer, and half a spoon of fertilizer. Both groups got some of the change.

Now here is Yara's real second group again: no fertilizer at all on any of its pots.

Is this group the control group? Yes, it is: it got none of the change.

Two groups: fertilizer, and no fertilizer. The second group is the control group.

Now consider Yara's seed test.

She puts one tray of bean seeds in a warm cupboard and one tray on the classroom shelf.

Is the tray on the shelf the control group? Yes, it is: it got none of the change, the warmth.

Yara's two trays of seeds: one in the warm cupboard, one on the classroom shelf.

Now consider a test of whether music helps children remember a list of words.

Two groups of children learning the same list of words. One group has a speaker playing music.

One group learns the list with music playing, and the other group learns the same list with no music.

Is the no-music group the control group? Yes, it is: it got none of the change.

But suppose the second group had learned the list with quiet music instead of none.

Is that group the control group? No, it is not: it got some of the change.

A little of the change is still the change. The control group gets none of it.