This is a one-credit course that can only be taken concurrently with Physics 191. The purpose of the course is to encourage students to work in small groups, supervised by a mentor, on problems from the course that will enhance understanding. Grading is Pass/Fail based on participation; a student must miss no more than two sessions to pass the course. Students will enroll in PLTL through an online application, but have the option of signing up to take it for credit. They can drop the course at any time. PLTL sessions guide students to become conscious of the problem-solving process, and to rigorously evaluate and revise those processes in light of the reasonableness of their results, rather than an answer key. Student feedback tells us that not having the problems graded gives students the freedom to explore the problems in depth, and to ask the key question "why is what I am doing wrong?" instead of "how do you do it?"