This is a simple houserule that changes the bonus a character receives for a natural 20 or a natural 1 on their skill tests.
Natural 20 Skill Test
Characters receive a +5 bonus to their result when rolling a natural 20 on a skill or attribute test.
Natural 1 Skill Test
Characters receive a -5 penalty to their result when rolling a natural 1 on a skill or attribute test.
Upon discussion with the party, many felt that critical results were fun, and less harmful than giving the DM the power to design a critical fail or critical success "story."