Must one wash the trousers if the impurity on the undergarment was wet and its traces did not appear on the trousers?

Wet impurities transfer to whatever they touch, whether it is wet or dry. If you touch a wet impurity, the part of the body that touched it must be washed because the impurity transfers. As for dry impurities, washing is not obligatory because they do not transfer. If an impurity transfers to a garment, it either becomes visible, in which case the garment becomes impure, or it does not appear: if it is slight and its effect is not visible, the garment does not become impure. However, if it is wet and clear, the more probable opinion is that it transfers and makes the garment impure. A child's urine is impure without exception, and during infancy and before solid food is consumed, sprinkling water on it suffices. But if a boy's garment is wet with impurity and touches another pure garment, if the wetness is evident, the garment it touches becomes impure. If it is slight and does not leave an effect, it does not become impure.