The brain typically struggles to do too many different things at once.
Now new research shows that the mind can easily deal with two separate tasks at the same time, because it can channel them into the two separate parts of the front of the brain.
However, when a third activity was introduced the mind became overloaded.
Accuracy declined and volunteers were much less able to carry out tasks, the study found.
The researchers used brain scans to identify what happens when our bodies attempt to do two things at once.
The hi-tech magnetic resonance imaging scans were used on 32 men and women given different tasks.
The scans focused on the left and right frontal lobes, which are involved in sophisticated mental functions.
When the volunteers tried to carry out two tasks at once the lobes divided the activities between them.
Signals on the left corresponded to the first task, while activity on the right to the second.
However, accuracy declined when the group tried to tackle three tasks at once, the findings, published in the journal Science, show.
Writing in the journal, the researchers, led by Dr Etienne Koechlin, from the Ecole Normale Superieure in Paris, say: "The two frontal lobes, which jointly drive the pursuit of a single goal, divide for driving concurrently the pursuit of two independent goals.
"This finding suggests that the human frontal function is limited to accurately driving the pursuit of two concurrent goals at one time.
“This capacity limit places a severe constraint bearing upon human higher cognition and may clarify several limitations in human decision-making and reasoning abilities."