truncate or delete plan_table

I try to delete plan_table . Its take a long time to delete from plan_table . Only 42 rews are in the plan_table. Then I cancel the statment and issue

truncate table plan_table;
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified

Why I got this error?
Can anybody explain me?
Thanks

tahiti.oracle.com

I got my answer

post the answer so other people can learn

An explain plan is executed in another session, no commit is issued.

Another good reason to set up plan_table as a Global Temp Table.