Observation and communication (or sharing) are two important science-process skills. Keep in mind these other skills as children become involved in outdoor explorations.
Children learn to classify by first noticing differences and similarities. Very young children can match identical pairs of objects or pictures. Eventually, they learn to sort objects that are alike but not identical, such as items of similar shapes or color. As children mature, they begin to understand that objects can belong to more than one category at a time. (Children often group objects according to their own categories rather than ones adults might expect or understand. Remember, there is no right or wrong way to sort a group of objects. The goal is that children are able to identify why something does or does not belong in a specific group.)
Counting, measuring, weighing, estimating, and comparing differences in size, space, and distance are all quantifying skills. Offer younger children opportunities to judge more or less, larger or smaller, shorter or taller, and so on. Older children can begin to find out how long, how far, how much more, and how much less.
The ability to predict what might happen based on past experiences is not only a critical science skill but an important life skill. It's one that develops with practice, so offer children lots of opportunities to make predictions. Even twos can make simple guesses about familiar activities: "What might happen if I touch that bubble?" At a more advanced level, a child might generalize that an untried object made of wood won't stick to her magnet because other wooden objects haven't in the past.
For young children, experimenting begins with open-ended exploration. They need opportunities to experience free investigation with materials that may or may not lead to identifiable discoveries before they are asked to follow adult-type instructions. From free exploration, children will, on their own, begin to direct their experiments and may also try out one another's experiments. Only then are they ready for adult-directed experiments.