Imagine you are on a guarding duty, without a chance to read, write or even sit down. And say you have a cloudy sky so you cannot even look at the milky way. What do you do? You can use the following as tips for IDF service.
Two friends of mine found a way to entertain themselves with arithmetics - YoavM computed and learned by heart squares of 3-digit numbers. AmirL was working hard to break his rifle's serial number into primes. To his disappointment, it was a product of just two primes, both 4-digit.
As for my personal account for guarding duties, my top achievement was discovering the Haley-Bopp comet. It was a cloudless night. More exactly, there was one cloud. I came out of the tower and saw it. "Ha, funny cloud" I thought. An hour later the lonely funny cloud still remained in the sky. OMG, it's a comet! I thought. The next day the funny cloud was there too, and with no TV or anything I could know for sure what we have here.