Parrots are smaller than hens. (Small is a one-syllable adjective, so we add -er).
Monkey's tail is longer than pig's tail. (Long is a one-syllable adjective, so we add -er).
Cows are fatter than horses. (Fat is a one-syllable adjective ending in a consonant preceded by a vowel, so we double the consonant and add -er).
Granny is shorter than Grandpa. (Short is a one-syllable adjective, so we add -er).
Apples are tastier than carrots. (Tasty is a two-syllable adjective ending in -y, so we change -y to -i and add -er).
Elephants are the biggest animals. (Big is a one-syllable adjective ending in a consonant preceded by a vowel, so we double the consonant and add -est for the superlative).
My brother is older than me. (Old is a one-syllable adjective, so we add -er).
Monkey are the cleverest animals. (Clever is a two-syllable adjective ending in -er, which often takes -est directly for the superlative).
Cliff is the strongest boy in our class. (Strong is a one-syllable adjective, so we add -est for the superlative).
Giraffe's neck is the longest. (Long is a one-syllable adjective, so we add -est for the superlative).