temp tables lock unrelated tables

[Michael Griffith]

Using SHOW PROCESSLIST or mysqladmin proc

Every time a table reports status of 'copying to tmp table' all other UPDATE queries 
are locked, even in unrelated tables.

For example:

Query #1: SELECT * FROM a JOIN B WHERE....   Status: copying to tmp table
Query #2: UPDATE C SET x=x+1 WHERE......    Status: locked

Even though table C is not used is query #2 it is locked until query #1 completed. 
This can cause huge problems when hundreds of quick updates are locked for more than a 
few seconds

Why does a tmp table on one table lock another?
How can I prevent this from happening?