Piping -- Sending the Output of a Command to the Input of Another

$ cal
    January 2001
Su Mo Tu We Th Fr Sa
    1  2  3  4  5  6
 7  8  9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31
                                     
$ cal | wc
      8      40     136
$ cal | wc | wc 
      1       3      24
$ cal | sed -e "s/10/MY BIRTHDAY/"
    January 2001
Su Mo Tu We Th Fr Sa
    1  2  3  4  5  6
 7  8  9 MY BIRTHDAY 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31
                                
$