Monday

Rocket thunders up the stairs and skids to a stop at the Windowsill. He presses two fingers to his wrist.

"My heart is pounding," he says. "I can feel it right here."

Raven looks up from the seed jar she is measuring. "What do you notice? Your wrist is nowhere near your heart."

"So something is carrying the thump all the way out here," Rocket says.

Nova hovers over the seedlings, her light soft and green. "A heart is one organ," she says. "What is an organ made of?"

Rocket frowns. "Cells? We spent four weeks on cells."

"Cells, and then something in between," Raven says, and she draws a ladder in the Windowsill Log with five rungs.

Living things are highly organized. They follow a hierarchy, a ladder of levels, on a scale from small to large.

All living things are made of cells. The cell is the smallest fundamental unit of structure and function in living organisms.

In most multicellular organisms, cells combine to make tissues. A tissue is a group of similar cells carrying out the same function.

Organs are collections of tissues grouped together based on a common function. Organs are present not only in animals but also in plants.

An organ system is a higher level of organization. It consists of functionally related organs.

Organisms are individual living entities. Each tree in a forest is an organism. A single-celled living thing is an organism too.

An organ system is a set of organs with related jobs. The circulatory system is one example.

It transports materials throughout the body and to and from the lungs. It includes organs such as the heart and the vessels.

The heart is an organ. The vessels are organs. Together they form a system that reaches every part of you.

When Rocket felt the thump at his wrist, he was feeling the circulatory system at work.

Good climbing. Tomorrow you will look closely at one organ, the heart, and the tissue it is built from.