Pass out a set of base-10 blocks to each student. If there are not enough to go around, divide children into small groups and give a set to each group.
Explain to the class what each block in the set stands for. The individual cubes are ones, the rods are tens and the blocks are hundreds. Show them that 10 cubes equal one rod and 10 rods equal one block.
Write the number "10" on the board. Ask children to show the number using only ones cubes. Once everyone has the correct amount of cubes showing, ask students if they have enough cubes to trade for a rod. They should answer yes and swap their 10 cubes for one rod.
Write a number between 11 and 19 on the board. As students to trade cubes for rods. They should end up with one rod and some cubes. Repeat with higher numbers, each time asking students to trade out so they end up with the least number of blocks, rods and cubes possible.
Tell students that today they will learn to regroup or "trade" in addition. Write "17 + 8" on the board. Instruct children to use their blocks to make 17 and 8. Using your own set of blocks, model for students trading 10 ones cubes for 1 rod. Make sure to completely remove the 10 cubes from the work area so they don't mix up in the final count.
Ask for a volunteer to say the answer and which blocks they have on their desk. The correct answer should be 25 and they should have two rods and five cubes. Repeat the process with several other regrouping addition problems.
Pass out paper to each student. This time the child will write the problem on his own and use pencil/paper method to solve the problem. Once again, write a problem on the board. Model carrying the ones over to the tens side as the students write the same on their paper. Remind them to think about the base-10 blocks in their minds as they work the problem. Give several problems for practice. Observe which students have mastered the skill and which ones still need help.