After each fence is built, write down the total number of toothpicks used. For example, the first fence used four toothpicks, the second fence used eight toothpicks, the third fence used twelve toothpicks. Have the child predict how many toothpicks would be used in the next fence if the pattern continued. Encourage him to build his fence and test the prediction.
Young children may struggle with small toothpicks or straws. Use building blocks or building logs for them to build their fences.