In order to get her 2nd grade students to memorize the poem written on the chalkboard, Mrs. Thyberg gives the students stickers for each stanza they can recite from memory. After earning 5 stickers, a...

In order to get her 2nd grade students to memorize the poem written on the chalkboard, Mrs. Thyberg gives the students stickers for each stanza they can recite from memory. After earning 5 stickers, a student gets to pick a prize out of the goody box. What is Mrs. Thyberg using to modify the children's behaviors?