Here's how I do it..
Make a Detective Jar. A Mason jar with a wide opening is the best to use. Print a label for it on a large piece of white paper that says "Detective Jar".
Make copies of the cards for your students. Explain that you'll be running a weekly contest and that you'll pull three cards from the jar at the end of the week. Be sure to have prizes for the winners. I order them online in bulk from the Oriental Trading Company.
Explain that there must be correct answers on the cards in order to win a prize.
Tell them they shouldn't cut the cards until they've entered the information on all cards. When they're finished they can cut the cards and fold them in half, then put them in the jar.
Hope you and your students enjoy these contests! Thanks for stopping by!