-
Click cell C4.
Note the formula in the Formula Bar:
='Numbers Per-Frog'!F8*Sales!B4

This means Excel is multiplying cell F8 in the Numbers Per-Frog sheet with January's frog sales.
When the formula in B4 was moved over to cell C4, Excel moved over the cells used in the formula, too:
From E8 to F8 in the Numbers Per-Frog sheet...

...and from A4 to B4 in the Sales sheet.
