Pre-treat two of the agar plates with the deodorant: Wet a cotton swab with the liquid from inside the spray or roll-on, then dab the plates gently so that they are covered with the wet solution. Let the deodorant solution soak into the agar completely.
Dip a fresh cotton swab into the bacterial control sample and streak a line onto one deodorant-treated and one untreated plate. (This will show whether a known sample of bacteria will grow in these environments.)
Take a sample from your armpit by wiping it with another cotton swab. Streak this sample onto one deodorant-treated plate. Take a second sample from the same source and streak it onto an untreated plate.
Optional: Make two more agar plates, pre-treat one with deodorant, and apply a sample from your friend’s armpit to both the deodorant-treated and the untreated plates.
Place the lids on your plates and label them so that you know which have deodorant applied and where the samples came from.
Store your plates upside down (with the agar on top) in a warm place overnight.
The next day, check out your plates for bacterial cultures (they'll look like wet, white spots). Where did bacteria grow?