Splitting a resource's time across multiple simultaneous tasks

I have a full-time resource that I need to work part-time on multiple tasks. How can I do this? I would like to only manipulate effort and let the tool decide how long the tasks take.

Ok, I got it. In the task pane where it lists the people working on the task, you can change the allocation. That is, if it says, “Fred | 85% of 85%”, you can click on the “85%” part and change it to, say, 43% of 85%. Now Fred can work on two things at the same time. The task duration extends appropriately.