How many 4-year-olds do you know that need to read stuff like I'm OK, You're OK, even if they could read? Humor is healthy (it reduces stress, amongst other things) and came naturally to us when we were kids. Then we grow up, take ourselves way too seriously and the notion of embarrassing ourselves paralyzes us.
You guys will risk your lives to protect others. Don't be afraid to try having a little fun. It's unpredictable. You'll sometimes bomb - which gives you another opportunity to make fun of yourself. But the pay off in training results is tremendous. And that's what it's about - our learners, not us.
Try this one
How do you make God laugh? Tell him your plans. (Attributed on the internet to "a wise man.")