Suppose a 17-year-old, Kendall, enters into an agreement to purchase a Harley Davidson from an ad she saw in the paper. She will pay $1,000/month until the bike is paid off. After making the third payment, Kendall turns 18 and decides to move out of state. She e-mails the seller of the motorcycle and tells him not to worry because even though she is moving, she still intends to make her monthly payments to purchase the bike. The above scenario is an example of what legal concept?