leza wang wrote:

Thanks for your replies. I have no idea from where it is leaking. i
touch the black U tube and I feel the water.

It sometimes takes a little patience and some careful looking and checking,
but you should be able to figure out where the water is coming from.

The water flows downhill. So, you start by drying everything off underneath
completely. Then, with water running in the sink, look underneath with a
flashlight. If you put piece of paper towel around the pipe that is coming
down (above the black pipe), you'll see if that gets wet.

If that gets wet, then the water is coming from above that -- maybe where
that "tailpiece" meets the sink -- the part where you are thinking of
putting the putty.

But, if the paper towel doesn't get wet, but the black pipe gets wet, the
leak is coming from below the paper towel. If that's the case, check around
the two parts that screw on to the black pipe (one on each end). You could
even put a small piece of paper towel on each of those. If either of these
is leaking, it could just be a plastic ring that goes inside the part that
screws on to the black pipe. There are different types of plastic rings --
"flat" ones, and ones that have a bevel. Look at what you have to see what
you would need if that is what you are replacing.

It is also even possible that there is a leak higher up (above the sink
drain) coming from around the faucet and running down the outside of the
sink that you would see from underneath the sink, and then running down onto
the tailpiece, then down onto the black pipe. Look for that too, and you
can use small pieces of paper towel to find that type of leak too.
Sometimes, it is hard to actually see the small drip of water, but the paper
towel piece will get wet if it is there.