Which Is It?

Group A:
leech
wine
calm
sherry
pillow

Group B:
staple
mead
cloak
sew
strife


Answer
The word "sew" can be put into Group A.

All of the words in Group A are types of trees, except that the first letter has been changed:
leech -> Beech
wine -> Pine
calm -> Palm
sherry -> Cherry
pillow -> Willow

and sew -> Yew.