Substitution is something your students are extremely familiar with. Use this to your advantage!
Start your lesson off with a real world example, something like this...
Teacher - Who in here plays basketball?
Johnny - I do!
Teacher - Do you start on the basketball team Johnny?
Johnny - Yes, I do!
(Everyone is very intrigued at this point. You have their attention!)
Teacher - Do you ever come out of the game Johnny?
Johnny - Sometimes...
Teacher - What do they call it when they take you out Johnny?
Johnny - Substitution, the person coming in for me is called a sub.
(They think he is teaching you something)
Teacher - So, does the player coming in for you play the same position as you?
Johnny - Yes, he would be taking over for me.
Teacher - So, ideally he would be a replacement and just as good as you?
Johnny - Ideally... Yes! (He will then make a smart remark about how good he is and the sub is never a good as the starter.)
Teacher - (If he does make a remark about their equality simply say) I understand Johnny, but IDEALLY he would be an equal replacement. Right?
Johnny - Yes...
Teacher - Interesting...
Then you can talk to them about a substitute teacher or however many other examples they need.
Once they start to understand that a substitute in any situation is ideally of equal value then you can go into teaching them substitution. This will give them a good solid foundation of what the word substitution means and how it is exactly the same in Algebra. We want to take something that is of the same value and substitute it in for something that needs to come out. Give them direct comparisons. If your variables are x and y then tell them y has to come out of the game what does he get replaced with of equal value? How can I make my equation have one variable?
Think about it realistically... When you substitute out the y (player) is the next of equal value? Not usually is x=y so what has to come in with x to make them the same value?
Be creative! Here are two cool little videos to help with your lesson...
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