When you multiply anything (positive) by a value less than 1, you get an amount less than whatever you started with.
A fraction less than 1 represents a part that is less than the whole. If the whole amount is some "whole number", a portion less than all of it will be smaller than that "whole number."
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If you need an example, here's one.
Multiply 10 by the fraction 8/10. The result is 8, which is less than 10.
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In number line terms, find your whole number on the number line. Now, divide the distance between that and zero into a number of equal parts that is equal to the denominator of your fraction. (For example, if your number is 39 and your fraction is 2/5, divide the distance from 0 to 39 into 5 equal parts.)
Now, starting from 0, count the number of parts equal to the numerator of your fraction. You will end up at a value less than the number you plotted on the number line to start with. (In the example case, each part is 7.8, so 2 of them is 15.6, which is less than halfway to 39, your original number.)
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"Whole numbers" include zero. Multplying zero by anything gives you zero, the same value you started with. This is the only case where a proper fraction of it will not be less than you started with.
