1. Wrap a bamboo mat with plastic wrap and place it on a flat surface or over a cutting board. Fill a small bowl with water and a little rice vinegar and place it next to the bamboo mat.
2. Lay a sheet of nori, shiny side facing down on the bamboo mat and place about 1/2 cup of rice in the center of the sheet. 
3. Wet your fingers with vinegar water and spread the rice evenly across the nori sheet. 
4. Sprinkle sesame seeds evenly across the rice (about 1 teaspoon).
5. Turn the nori sheet over so the rice is facing down and carefully slide the sheet close to the edge of the bottom of the bamboo mat.
6. Place the cucumber, imitation crab, and avocado at the bottom of the nori sheet.
7. Grab the bottom edges of the mat with your thumbs while holding on to the toppings with your other fingers. Roll it into a tight cylinder, tucking the fillings in firmly and slightly lift the edge of the bamboo mat.
8. Roll it forward while using a little pressure but not so much that the filling spills out. Let go of the mat when you get to the other end.
9.  Cut the roll in half and each half into thirds.
10. Serve with soy sauce and wasabi on the side.

For spicy California rolls: Start by spreading the spicy mayonnaise across the bottom of the nori sheet (about 1 teaspoon or a little more if you want more flavor) and top with cucumber, imitation crab, and avocado. Follow the rest of the recipe as written.