From the course: After Effects CS5 Essential Training

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What are expressions?

What are expressions?

Expressions are just another way to control properties of layers in After Effects. Now they look like a programming language and they are kind of our programming language, but they are not that challenging. So, a lot of new users are intimidated by expressions. You need not be. So, let's go ahead and open up this Left Wheel layer properties. Go ahead and press T for Opacity. The way we set expressions is by holding down the Option key on the Mac or the Alt key on the PC when we click the stopwatch. Then we get this field that allows us to input information. So, it's just looking for a number here. That's basically all expressions are. I could type in 75 and if I press Enter on the numeric keypad or simply click outside of this field to accept that value, then that's what happens. It just turns the Opacity to 75%. So, as you could see here, Opacity, the value turns red and the red indicates that the value here is being received or calculated based on the expression. So, I might say…

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