This is a really old one but since the time it was first gestated it’s had two updates. In short, let’s assume that New York and San Francisco are just seven days apart travelling by train. In that case, let’s also assume trains start from both ends every day at noon. How many trains coming in an opposite direction will a train leaving New York meet before it arrives at its destination at San Francisco?

Right, now we come to the Matrix red and blue pills part which I love. There is of course one straight answer and most of you are going to get it. Clap clap clap. There’s also one slightly skewed one which again most of you might get. Clap clap. Then there’s a third. That should nominate you for the Ig Nobel Prize. Clap.

**THROUGHPUT**

(The new problem was: “You reach a river at point A and wish to know the width across to B. Since you have no means of crossing the river, what is the easiest way of finding the width?

Measure any convenient distance along the bank from A. to C, say 40 metres. Then measure any distance perpendicularly to D, say 12 metres. Now sight along DB and find the point E. You can then measure the distance from A to E, which will here be 24 metres, and from E to C, which will be 16 metres. Now AB:DC = AE:EC, from which it is evident that AB, the width of the river, must be 18 metres. -- Dhruv Narayan, dhruv510@gmail.com

We need to use some trigonometry to determine the width of the canal. I am at point A and the opposite point on the river bank is B. I mark a point from A on the river bank say at 100 meters to right -- ie, point C. Now I measure the angle ACB. The width of river is = Tan of angle ACB multiplied by 100 in metres. -- U N Murthy, u_n_murthy@rediffmail.com

Let us say you are standing on the south bank of the river. Mark that point as A. Now locate a point exactly opposite on the north bank and mark that point as B. Now walk along the south bank towards east (perpendicular to AB) for 50 feet and mark that point as C. Walk further another 50 feet and mark that point as D. Now turn 90 degrees and walk southwards to a point E where the points B, C and E lie in a straight line. Now you have got two triangles ABC and CDE which are equal. Here DE = AB = width of the river. -- Dr P Gnanaseharan, gnanam.chithrabanu@gmail.com

(The second one was: “When liquid nitrogen spills on the floor, the gaseous nitrogen from it stays at the bottom -- at least, for a while even though N2 is lighter than air. Why? Also, if ozone is denser than air, why is it up there in the stratosphere?”)

Liquid nitrogen is cold and when it spills, it becomes denser than the warm air surrounding it. Hence, the gaseous nitrogen stays at the bottom until it becomes warm because of the surrounding air. When it becomes warmer than the surrounding temperature, it begins to move upwards. Ozone forms in the stratosphere when certain trace gases like oxygen in the upper atmosphere interact with large amounts of UV light from the sun. It only exists, in the stratosphere, because ozone is highly unstable and not due to being denser than air. -- Shashi Shekher Thakur, shashishekher@yahoo.com

The reason is because it is created there. UV radiation from the sun knocks off the oxygen molecules by photolysis, creating ozone. But ozone is highly unstable and reactive to be available in its natural state lower in the atmosphere. -- Saifuddin S F Khomosi, Dubai

When liquid nitrogen spills it evaporates immediately as the floor is at room temperature, which is way higher than liquid Nitrogen's boiling point of -196° C. This sudden evaporation causes it to form a layer between itself and the floor, called the Leidenfrost effect. This causes liquid Nitrogen to skittle around and slows down the evaporation process and Nitrogen stays at the bottom for a while. -- Balagopalan Nair K, balagopalannair@gmail.com

**BUT GOOGLE THIS NOW**

1. How can you draw two straight lines on a clock face so that the sums of the numbers in each part are equal? (You can submit a drawing if you need to.)

2. Four ballistic missiles are positioned at the corners of a square 20 kms on each side. All are launched at the same with each homing in on the one on its left at 1 km per second. How long will before they meet?

**Mukul Sharma
Sharma is a scriptwriter and former editor of Science Today magazine.
mukul.mindsport@gmail.com**