I learned this at a workshop and my fourth grade students love it. We've used it at our school with grades 3-6.
First explain that you are going to play a game called pico, fermi, zelch. You use problem solving strategies to guess the two digit number that the teacher or student has thought of.
Then explain that pico means you have one digit right in the right place. Example if your number is 34 and they guess 39. You would say pico, because the 3 is in the right place. Next explain that fermi means you have one digit right but in the wrong place. Example still using the number 34, they guess 93. You respond with fermi. They have one correct digit but it's in the wrong place. Zelch means no correct digits.
The first few times you play this game just let the students figure out the numbers on their own. Later you may want to suggest some problem solving methods for figuring out the number. The first time we played this game I had a few students that would write the numbers guessed down. An easy way to solve the number is to write you a number line from 0 to 9. When each number is guessed write it down and go to your number line. For example if my number is still 34 and they guess 76(which is zelch) they can mark out the 7 and 6 on their number line. When they get a peco, they can then keep up with which numbers are correct. Eventually they will catch on to the problem solving techniques and have war over trying to figure out the number before their friends do.