random sampling from a data.frame

I have a stimuli set (just an example, not a real data set):

df<- data.frame(A = c("dog", "cat", "bear", "zebra), 
B = c("mosquito", "cockroach", "fly", "worm")

and I would like to randomly sample one from this data set. This should be an easy one, but I cannot seem to find a way.

sample(df, size = 1, replace = T) 

returns one whole column, not just one (i.e. just "dog").

Does anyone happen to know how I can solve this problem?

Thank you.