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 “Each day, I make up a rule for myself. Yesterday, I threw back the ﬁrst ﬁsh I caught, kept the second, threw back the third, kept the fourth...”

“I know, you threw back Fish 1, Fish 3, Fish 5 ........... You threw back the odd-numbered ﬁsh!”

“That’s right. Today, I kept the ﬁrst ﬁsh, threw back the second, kept the third, threw back the fourth...”

“All the even-numbered ones went back into the river. Poor odd-numbered ones!”

“Tomorrow, I’m going to need a little more money. So I’ll have a new system. Fish numbers 2, 3, 5, 7, 11, 13, 17, 19, 23, and so on will go back into the river. The rest will get into my basket.

“Grandpa, how many ﬁsh do you sell each day?” “About 50.”

1 is an odd number. 2 is an even number.

Numbers that cannot be divided by 2 are called odd numbers.

Numbers that can be divided by 2 are called even numbers. 18