Leave the gallon, quart, pint and cup bottles or measuring units unlabeled. Show the class the four measurements and ask if they can identify which bottle or measuring item is a gallon. Allow the class to guess and then tell them the correct answer.
Explain all the measuring units. Clarify that two cups make one pint, two pints in one quart and four quarts in one gallon.
Give rules for how to get to one gallon from the various smaller units. For example, tell the students that there are 16 cups in one gallon and that to get one gallon from cups, the student must add 16 cups.
Provide the class with worksheets presenting conversion problems and math rules. Give this as homework and review the next day. If the students continue to struggle, give more examples of how to convert the units and identify a gallon.