Ask a student to pick a card from the alphabet deck. My kindergarteners love it when I say.."Pick a card, any card." Students display their card to the class and announce what letter was picked. Then the student takes the pointer and points to each letter on the alphabet line and the class sings up to the letter picked and then stops. My students like the abruptness of stoppping at the chosen letter..and try hard NOT to accidently sing further. I always end the session by singing all the way to Z.