Question

where does matter go when a bear loses weight?

Explanation:

When a bear loses weight, the matter (stored as fat, carbohydrates, etc.) is metabolized. Through cellular respiration, the organic molecules are broken down. The carbon in these molecules is released as carbon dioxide ($\ce{CO_2}$) when the bear exhales, and the hydrogen and oxygen are released as water (either exhaled as water vapor, excreted in urine, or sweat). So the matter is converted to $\ce{CO_2}$ and $\ce{H_2O}$ and released from the bear's body (via respiration, excretion, etc.).

Answer:

When a bear loses weight, the matter (from stored energy sources like fat) is metabolized. Through cellular respiration, it is converted to carbon dioxide (exhaled) and water (excreted or released as vapor), so the matter leaves the bear's body as carbon dioxide (via breathing) and water (via excretion, respiration - related vapor, etc.).