Time needed: 10 to 15 min
Why? The easiest way to learn is, if students can touch things. With that method you are giving them a homework to decide which object is match to the taught subject.
When? Any time the teacher need practical connections to the theoretic topic.
How? The teacher is telling to the students to bring one object which is connected to the taught subject for the next lesson. In the lesson the teacher can open a discussion always connected to the objects the students brought over. The teacher is able to ask the students why they brought that special object and the other students are able to participate the knowledge from each other and everyone can touch these things and will learn in a practical way.
Any risks? There is no risks at all, the method guarantees two possibilities of learning, one while choosing the object by self and the second one inside the class while evaluation of the brought things.