Ambiguous?

1) query
foreach($varenr_inn_pa_lager as $vnr){
            include('../../tilkobling.php');
            $sql = "SELECT * FROM exan,dynamit WHERE varenr = '$vnr' LIMIT
1";
            $resultat = mysql_query($sql, $tilkobling) or
die(mysql_error());

2) Result:
Column 'varenr' in where clause is ambiguous

3) Tables in MySQL database

mysql> SELECT * FROM exan;
+------------+-------------+-------+----------+----------+-------------+
| leverandor | valgt_lager | un_nr | varenavn | varenr   | kg_pa_lager |
+------------+-------------+-------+----------+----------+-------------+

mysql> SELECT * FROM dynamit;
+------------+-------------+---------------+--------+-------+---------------------+------------+-------------+
| leverandor | valgt_lager | type          | dim_mm | un_nr |
varenavn            | varenr     | kg_pa_lager |
+------------+-------------+---------------+--------+-------+---------------------+------------+-------------+

4) My question:
What means with "ambiguous" here?

Thanks.
Karl