I am sure most kids around the world have this "assignment" of growing plants to learn a few tricks, so here is quick review of what growing corn can teach you and your kids.
Start a popcorn seed
Just any regular popcorn seed will do. Put it in a wet cotton. It will sprout after a few days. Of note the sprouting happens with water only. You don't need light or nutrients, proving that the seed is a little self bootstrapping entity. To show this, grow 3 seeds. One with water and light, one with light only, another one with water only. The one with light only will not sprout. The only with water only will.
Expose the Sprout to light: Photosynthesis
In a little cup or glass, at this stage no need for fancy nutrient or anything. Put the plant in sunlight and it will start photosynthesis. At this stage you can compare with a sprout left in the dark. The one in the dark, that you have kept in the dark will not grow. Kids understand they need "light". However what is interesting, if you leave the plant in cotton is that the kids ask "where does the food come from". I struggled a bit, because frankly, I had never properly mapped photosynthesis.
Of course the food doesn't come from the soil (in this case cotton) but from the air as in C02 + sunlight = C + 02. The kids can see that the plant grows and it gets its carbon needs from the air. You would be amazed at the amount of adults that answer "the food comes from the soil".
Explaining the carbon cycle to the kids is then an easy mental step for them. Plants do C02 + energy = C + 02 and humans : C + 02= CO2. They intuitively understand this step. Leo asks "then pollution is good for plants". Explain that plants indeed offset carbon emissions, this is a good time to get into why C02 in surplus is not a good thing. Notion of 'ecosystem' can be touched upon.
Plant the sprout
Eventually the sprout needs to grow out of the cotton and needs to be planted. The root system grows and that goes better with real soil. The kids understand that the root is a way for the plant to get water. Keep it in sunlight and it will grow. At this stage the kids focus on "taking care" this took a couple of weeks in our household. They come every day and check the progress and "feed" the plant some water. They learn to care for living things.
Nutrients bigger pots
At some point, you need a bigger pot, the one you see in the picture is where we are at. At this stage, nutrients are helpful. The kids understand that water and light are good but you need something more.
the magic start happening and the plant grows a daddy part (the wee-wee on top) and a mommy part the pink fluffy stuff. If you look closely you see 2 such fluffs, one to right still pink and one to the left at the same height, kind of crumpled and orangey. Explain mommy and daddy in absolute terms. It is freeing to discuss sexual reproduction in precise scientific terms without embarrassing everyone.
Since we didn't know the first thing about corn, Elvira and I googled vast amounts, she was the one digging the information about the plants, including the interesting following bit.
From google we understood that the mommy from one plant could not be pollinated by the daddy from the same plant. Explain the cousin thing from the South and why that is not a good thing (notions of DNA can be introduced). Elvira found the additional bit of information that in a field you need 10 plants to achieve cross pollination. That is because statistically the wind will take the seed and encounter another plant in its vicinity. At two plants the wind alone will not suffice.
Armed with this information, what we did was essentially to rub the daddy parts from one plant to the pink parts in another plant. The kids are very aware of the fact that they are MANUALLY moving the seeds around and are assisting the plant in getting pregnant. They took it so seriously that if you look closely you will see that the left plant is missing the daddy part. OUCH. Eventually we taped the daddy part to the left mommy part.
This last part is a bit abstract and went above the heads of the 8 year olds but I think the 11 year old got it. The question is "why is it a no-no to self pollinate"? re-explain the in-breeding cousin situation and marrying your sister. They get that. What they may not get is that 'natural selection' will select against that. Explain that diversity of genes yields strength by augmenting your capabilities. I talked in terms of nintendo DS game (see post on Trebuchet) the twins realize you need an army with various types, just one type, say infantry or siege weaponry, would be destroyed. Introduce the concept of 'fitness'. The final step is to get them to understand that plants that RANDOMLY develop this trait of cross-pollination will survive and thrive. This is an abstract thought in time (they need to imagine several generations and numbers game).
The plant is pregnant.
From the picture you can see that the left pink is shriveled and orangey (you can barely tell , but anyway). The cob is also there and the plant is 'pregnant', its 'belly' is growing and that is a source of immense joy for the kids.
They absolutely love it and are looking forward to eating the corn.
Priceless Good Clean Fun for the scientific dad.