The basic premise is this, when you are with a group of people who know about the game, at any point one person can sit down on the floor. This can be done as discreetly as they like. When the others in the group notice this, they must then also sit down on the floor.
The last person left standing up is the tall silly monkey, and must perform as one for the entertainment of the group. This should include arm waving, facial expressions and a range of monkey noises.
Once this has happened, life returns to normal until the next person chooses to sit down.
Note - there will be some clever person who thinks that by remaining seated, they avoid the risk of becoming the tall silly monkey. To avoid this, explain that anyone who has not stood up or sat back in their chair in between, they automatically become a small silly monkey, and must perform a similar show while sitting on the floor.
This game can really take over an evening, with people nervously looking out for the slightest sign of someone preparing to sit on the floor. There's nothing like an air of mistrust to make for a fun night!