Sit your baby in a high chair or on your lap at a table and give them a large, safe object.
When they drop it, say what happened — 'it fell down!' — and hand it straight back.
Do this as many times as they want. It looks like a game about you fetching things; from their side it is an experiment being repeated for reliability.
Dropping the same object over and over is a baby running a controlled experiment: same action, same result, every time. Handing it back is what supplies the repetition, and repetition is how the rule 'my actions cause effects' gets established.