1. If the boxes have any holes in them, cover over the holes by sticking pieces of cardboard or paper over them so the boxes are dark inside when closed.
2. Cut a large window in the side of one of the boxes. Leave the other box intact.
3. Line the bottom of each tray with a piece of kitchen roll folded to fit, or with a layer of cotton wool.
4. Sprinkle a similar number of seeds in each tray. It doesn't matter too much if you use a generous number of seeds or a light sprinkling.^{22 2}More seeds means more cress to eat later! But fewer seeds means you can do the experiment again - I usually manage to get two runs of the whole thing out of one packet of seeds.

5. Pour water into three of the trays. Don't put any water in the last one.

   Use enough water to thoroughly soak the kitchen roll or cotton wool, but not so much that there's a lot of water sloshing around the bottom of the trays. You can always tip out any excess.

6. Put one of the watered trays inside the box without the window. Put another of the watered tray inside the box with the window. Put both the boxes and the remaining two trays (one watered, one dry) somewhere that gets daylight.^{33 3}Strictly speaking, for a full controlled trial we ought to have one more: a tray in the dark and without any water. It will do exactly the same as the tray without water in daylight: namely, nothing at all! If you want to be more thoroughly scientific about this, and you have enough seeds and boxes, go ahead and add this extra tray. It doesn't need to be direct sunlight. Near a window is good, but anywhere that gets some natural light shoule be fine.

   If you put the trays near a window, or anywhere that gets more light from one side than the other, make sure you rotate the trays that are in the open at least once a day for best results.

1. Each day, take the trays out of the boxes (remember which tray came from which box), and have a look at what's happening to the cress.
2. Prompt your children to observe and describe what they see, especially the differences between the cress in the different trays. Encourage them to think about why the seeds are doing what they've seen, and why the cress in different trays is growing differently. See What to talk about for suggestions.
3. Water the same 3 trays as before (the two trays from the boxes, and one of the trays in the open). Don't water the last tray (the second tray in the open).
4. Put the trays back into the same boxes they came from, and close the boxes.
5. Rotate the trays that are in the open, so they get light from all sides.

After about a week, there should be obvious differences between the cress in the different trays. Once you've finished this part of the experiment, you can eat the cress from the tray that had both water and light if you like!

Prompt your children to observe what's happening to the cress and think about why that might be happening, by asking questions such as:

Like us, plants need water to survive. Because water is so essential to life, seeds won't even germinate (start to grow) without it; they'd quickly die from lack of water if they did. So seed germination is triggered by water:

The seeds in the tray that doesn't have any water will just sit there, doing nothing. Just like they did in the packet:

When you give them some water in the second half of the experiment, they start to sprout:

Whereas we need to eat food to get energy to survive, plants get their energy from light. Chlorophyll (the green stuff in plants) uses energy from light to turn carbon dioxide from the air and water the plant gets through its roots, into sugar which the plants then use as an energy source: a process is called photosynthesis. The seeds growing in daylight quickly turn green, as they make clorophyll to start using the light energy to make sugar:

The cress inside the completely closed box isn't getting any light, so clorophyll would be no use to it. It's yellowish-white because it hasn't produced any green clorophyll yet:

As well as being yellowish-white instead of green, the cress in the dark grows faster and taller than the cress in the daylight, because it's trying to reach daylight as quickly as possible.

Usually, if seeds find themselves in the dark, it's because they're underground. So the first thing they do is grow as fast as possible upwards, until they reach the surface and daylight and start producing clorophyll.

The cress in the box with the window gets plenty of light through the hole we cut in the side of the box. So it grows and turns green like the cress growing in the open:

But after a few days, the cress in the box with the window will be noticably bent over towards the side of the box with the window, where the light is coming from:

The cress is trying to get as much light as possible, so it can generate plenty of energy by photosynthesis. So it grows towards the light, and bends towards the window. Whereas (if you remember to rotate it regularly!), the cress growing in the open is getting light from all sides, so there's no reason for it to bend one way or the other, and it grows straight:

It doesn't matter whether it's about discovering what seeds need to grow, or about discovering the Higgs boson: science starts with noticing something interesting, then asking "why?"^{55 5}Which young children often excel at! Learning what plants need to grow is interesting. But learning to carefully observe what's happening, learning to then go onto think about why it happened and try come up with some explanation, is far more important. These are key skills of any scientist. Science isn't a collection of facts; it's a way of asking questions that leads to new facts and new discoveries.

It's more important to encourage your children to look carefully and tell you what they notice about the cress for themselves, than to get them to spot the "right" things. They may need some help and guidance to notice some things. But helping and encouraging them to spot things for themselves, rather than pointing it out to them, encourages an interest in observing things around them.

Young children will likely need a little more help in thinking about why the cress did what it did. But again, encourage them to think about it for themselves, rather than just explaining what's going on. Children are creative, and directing that creativity towards trying to come up with explanations of things they observe, however wrong or fanciful or "non-scientific", is a more significant step towards thinking like a scientst than learning why cress grows faster in the dark.

If we're trying to work out what things plants need to grow, it's no good just planting seeds, watering them, giving them daylight, and seeing them grow. It could be they need water and light to grow. Or it could be they just need water. Or just light. Or maybe they don't actually need either. If you'd planted them in soil, you might think they needed soil to grow. And how do you know the crucial thing wasn't the water or the daylight, but the fact that you happened to plant them in a blue flowerpot?

To tease apart which factors are important, and what effect each one has, we need to test them one at a time. By comparing the two trays in daylight, one with water and one without, keeping everything else the same, we can see what effect water has on the seeds. By comparing the tray in the closed box (which we watered) with the tray in the daylight which also got water, we can see what effect light has on the seeds, all else being equal. By comparing the tray in the box with a window cut out, with the tray in the open which we rotated regularly to make sure it got light from all sides, we can see whether the direction the light is coming from has any effect.

Wherever possible, scientific experiments need a control group: one (or more) other runs of the same experiment changing just one factor at a time, to see if that particular factor made any difference, and exactly what difference it made.

The idea of a controlled trial may be too complex to explain to young children directly. But seeing it in action is a good way of introducing the idea from early on. Encourage your children to compare two trays of cress at a time, and spot the differences between them. Then get them to remember what you did differently to the two trays when you first set them up, and each day after that. Finally, encourange them to think about what effect that difference might have had, and discuss with them their ideas for why it might have had that effect. This already gets across the idea of comparing one factor at a time (water, light, and light direction in this experiment), and working out what effect each individual factor has, separately from the other factors.