Today’s puzzles have been suggested by he Department of Physics at Oxford university, for reasons that will become clear below.

They kick off with a question that could change your life, that is, if you are always burning your mouth on hot coffee.

**1. Cuppa conundrum**

*You are in a rush to work. You pour yourself a cup of black coffee, but it is too hot. You intend to add a fixed amount of cold milk to it, but you know that even after that, the coffee will need to cool down for a few minutes before you can drink it.*

*In which case does the coffee cool down more:*

*1) Add milk right away, then wait a few minutes before drinking.**2) Wait a few minutes, then add milk just before drinking.*

**2. Fly away**

*A fly has landed at the bottom of a glass that is sitting on a very sensitive digital scale. All of a sudden the fly takes off. What happens to the readout on the scale?*

1) it goes up

2) it goes down

**3. Bounding Balls**

*A weightless string connects two identical metal balls. The middle of the string is over the edge of a table. You let go of both balls at the same time. What will happen sooner: ball 1 flies off the table or ball 2 hits the side of the table?*

**4. Fun fraction**

*Prove (1/2) x (3/4) x …x (99/100) < 1/10*

*It looks complicated, but there is a two line solution.*

Yes, I know. The last one is maths, not physics, but it allows me to segue into the reason I have posted these puzzles today.

If you are a smart sixth former at a state school in the UK, then you are eligible to take part in a fantastic free course run by Oxford university that will teach you mathematics and physics at a deeper level than the A-level curriculum, preparing you for studying these subjects at university.

If you are not a smart sixth former who loves the hard sciences, but you know someone who is, please let them know about it. To apply you need to register and submit some work by October 16.

The new course is called the Comprehensive Oxford Mathematics and Physics Online School (COMPOS), and is the brainchild of Alexander Lvovsky, Professor of Physics at Oxford. He suggested the puzzles printed above.

He adds that if you can solve the first two puzzles above, you are probably at the right level for the course. The final two questions are a bit more complicated, requiring a spark of ingenuity.

I’ll be back at 5pm UK with the answers. Please NO SPOILERS

UPDATE: The solutions are available here

*I set a puzzle here every two weeks on a Monday. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.*

*I’m the author of several books of maths and puzzles, and also the co-author with Ben Lyttleton of the children’s book series Football School. The latest in the Football School series is *The Greatest Ever Quiz Book, out now!

*I give school talks about maths and puzzles (online and in person). If your school is interested please get in touch.*