How to select only certain rows when making a new dataframe?

Dear R-users,

I have two data frames. The "FIRST" data frame has
100 rows, the
"SECOND" data frame has only 50 rows.

The data frames have different variables in columns
("VAR1," "VAR2,"
etc) but they share a column called "ID" that
contains a unique
identifer linking the two data frames.

I would like to make a "THIRD" data frame containing
just the rows of
the "FIRST" data frame that match the rows (on "ID")
in the "SECOND"
data frame.

THIRD=data.frame(FIRST$ID,FIRST$VAR1,FIRST$VAR2)

How can I modify this line to include in "THIRD"
just the rows in
"FIRST" that match the rows in "SECOND"? (i.e.,
contain the same value
in the shared "ID" column).

Thanks! Mark