Happiness
Your monkeys grow only when they are happy. One way to make your monkey happy is to rub his belly over and over with your stylus until he smiles. If you do this rapidly, he will grow into an adult monkey more quickly.
Hearts
When your monkey has three full hearts, he will grow into an adult monkey. To make him gain three hearts, rub him until they appear at the top of the screen. Stop rubbing him immediately, and a message will appear that he is fully grown.
Rapid Growth
Monkeys can grow in as quickly as 60 seconds. To grow your pet monkey this quickly, pet it as rapidly as you can for 45 seconds. When he stands up, pet him vertically rather than horizontally. He will almost immediately grow into an adult monkey.
Rubbing
The key to growing your monkey is sliding the stylus over the monkey's belly as fast as you can. The faster you move the stylus, the quicker your monkey will grow into an adult. If you move the stylus slowly, the monkey won't grow as fast.