Keeping formula values constant

Hey Guys -

This is going to be a tough one to explain but I'll give it a try. . . I'm creating a spreadsheet to add up my mileage. I have one cell dedicated to the current cost per mile figure and use it as a reference in a formula for other cells. I then type in my mileage for the day, multiply it by my Cost per mile cell and it tells me what I've made for the day. I turn in my numbers at the end of the month so at the bottom of the spreadsheet i total each days travel. When I copy and paste or drag the little circle in the lower right hand side of each cell, the formula is added to the new cells but each cell is added dynamically. I want the daily mileage to be dynamic but the cost per mile cell to remain static. This probably makes no sense so let me illustrate:

Cost per mile cell = B3

Col. A Col. B
Row 1 : [ mileage ] [cell 1A * cell B3]


when i copy or drag i get this:

Col. A Col. B
Row 1 : [ mileage ] [cell 1A * cell B3]
Row 2 : [ mileage ] [cell 2A * cell B4]
Row 3 : [ mileage ] [cell 3A * cell B5]
Row 4 : [ mileage ] [cell 4A * cell B6]


I want this:

Col. A Col. B
Row 1 : [ mileage ] [cell 1A * cell B3]
Row 2 : [ mileage ] [cell 2A * cell B3]
Row 3 : [ mileage ] [cell 3A * cell B3]
Row 4 : [ mileage ] [cell 4A * cell B3]

I want B3 to stay as the reference cell in each formula. I know there's something I put in front of B3 to keep it static but I cant remember what it is. I hope this makes sense and thanks in advance for your help!