You have to find the right keys! (Because pianos have keys)

25, 27.

There are two alternate sequences: start at 10 and add 3, 5, 7; start at 50 and deduct 5, 7, 9.

U and I.

You light the match first!

A penguin.

QT (cutey)

The donkey is on one side, the carrot is on the other.

You can hang the iron rods on a string and watch which one turns to the north (or hang just one rod).
Another solution: take one rod and touch with its end the middle of the second rod. If they get closer, then you have a magnet in your hand

Weights.

I am... the crook of an elbow!

The crook is called a crook (obviously), and as your arms are on the side of your body, the insides of your elbows most nearly always have an inside angle on you; it's not psychological at all! And for every arm is an elbow, so two elbows means two crooks of an elbow per person.

A clown.

The Police didn't tell the man where the crime scene was, but the man knew.