Which is NOT a good blocking factor?

As discussed in the video, the purpose of blocking an experiment is to make the experimental groups more like one another. Groups are blocked by a variable that is known to introduce variability that will affect the outcome of the experiment but is not of interest to study in the experiment itself.

A rule of thumb in experimental design is often "block what you can, randomize what you cannot", which means you should aim to block the effects you can control for (e.g. sex) and randomize on those you cannot (e.g. smoking status). Variability inside a block is expected to be fairly small, but variability between blocks will be larger.

Which of the following would NOT make a good blocking factor for an experiment?