Regression splines often give superior results compared to polynomial regression. Unlike polynomials, which must use a high degree (the exponent in the highest monomial term, e.g., $X^{15}$) to produce flexible fits, splines introduce flexibility by increasing the number of knots but keeping the degree fixed. As shown in the accompanying figure, polynomials can exhibit unexpected and wild behavior, especially near the tails. This is not preferred for larger range predictions.