Their first invention I am sure was teaching monkeys to juggle barrels because in the first episode where they built the rollercoaster Candace was telling her mom that they built a rollercoaster her mom sinabi "I believe we have had this conversation." Candace sinabi ," What do you mean?" And Her mom then sinabi that she did the exact same thing when they were teaching monkeys to juggel barrels, but as in building I would say the rollercoaster. If you mean anything it would be teaching monkeys to juggle barrels.
