Presentation ideas

How big is foo?

Ok, hands out!
How big is a ... ?

The idea is that this dialog might permit drilling a group on size. Both on units of length, and on the sizes of things. A length is specified, and everyone... hand waves.

"Ok, hands out! How big is 100 millimeters? 500? 1 centimeter? Ok, in micrometers, how big is a red blood cell?"

Range of about 1 mm through 1 m. A bit less for smaller kids.

Building familiarity with the metric length scale, a plausible spread might be 1 mm, 1 cm, 5, 10, 25, 50, 75, 100 cm.

Caveat: I've never actually tried this with a group.

Perhaps "Hands up!" rather than "Hands out!"?

"tick, tick", group answer

The objective is to have a fast-paced dialog in which everyone is actively trying, and generally succeeding, in answering questions. Calling on individuals won't do it. A question is asked, one "tick"s to provide time for thought, and based on visible progress feedback from the class, uses body language, tone, etc, to indicate the nearing group answer, and to trigger it. Something like this is the only way I can see to even come close to packing in the amount of practice and learing need to make this class work.