Two Houses

In plants (as in animals), the male parts contribute pollen (sperm), the female parts contribute the ovule (egg); the female grows the seed - the product of the combination of pollen and ovule. (Details about flower parts here.)
The vast majority of  flowering plants have male and female parts on the same flower.
Some flowering plants have separate male and female flowers on the same plant (monoecious); e.g., Mexican Blue Oak (Quercus oblongifolia).
And some flowering plants have male flowers and female flowers on separate plants (dioecious).
Let's look at some dioecious plants (and their fruits).

Photos by Matt Ball 10/23/2012

The Four-wing Saltbush (Atriplex canescens) grows separate plants for each sex. The fruits (above) have 4 wings, hence the common name. Ask your kid(s): "These are the fruits, which sex is this plant (above)?"

Male flowers never grow fruits. What do they contribute?

Same species, different sexes. Which one is which? 

There are loads of Four-wing Saltbush plants (of both sexes) at the Rillito River Park, starting at Craycroft,  on the north side and heading west. (I'm sure there are more elsewhere, but this is where I've been.) You can very easily tell the sexes apart now.The female plants are absolutely laden with fruits.
Loads of Desert Broom plants along the Rillito as well...