To begin the unit on comparting fractions with different denominators, the fourth-grade teacher can BEST link this new knowledge with prior knowledge by saying -

A. "Last year in third grade you learned something about fractions. Who can tell me what you learned?"
B. "If I have one half of a bag of popcorn and you have a full bag of popcorn, who has the most? What if you eat half of your popcorn?"
C. "I've brought in this pizza today and will be using it to help you compare fractions with different denominators. Let's look at the whole pizza first. If I cut it into two pieces, what will my denominator be? What if I cut it into four pieces?"
D. "My objective today is for you to be able to compare fractions with different denominators. Does anyone have a question so far?"