children CLUEAdd -ing to the infinitive → playing

 girls CLUEAdd -ing to the infinitive → screaming

 cowboys CLUEAdd -ing to the infinitive and drop silent -e → dancing

 ducks CLUEAdd -ing to the infinitive and double short consonant -m → swimming

 babies CLUEAdd -ing to the infinitive → crying

 water CLUEAdd -ing to the infinitive and double short consonant -n → running

 teachers CLUEAdd -ing to the infinitive → singing

 leaves CLUEAdd -ing to the infinitive → falling

 people CLUEChange -ie to -y, then add - ing to the infinitive → lying