My pastor’s family was over for dinner and their sons tend to be a bit of a germaphobes. The boys (10 and 12 years old) were coming in from outside and their mom was concerned that they would not wash their hands properly. So I offered to make sure they would do it right.
I had the boys come over and show me their palms. I then promptly (pretended) to spit in their hands. Looks of shock, horror and pained expressions of “I can’t believe I just saw that.” as the boys went “eeeewwwwww” and headed off to the bathroom.
I turned to their mom and said “Now I KNOW that they will get their hands clean”. Since then, their mom threatens to have me check their hands.
that’s both funny & effective.